Pseudo-four color twisting ball display

ABSTRACT

A seven-segment ball for an electrical twisting ball display device made up of spheroidal balls rotatably disposed in an elastomer substrate. The device built with the seven-segment balls can provide, for example, two fully saturated colors, two partially saturated colors, and a background color, such as white. The ball is composed of seven segments arrayed substantially parallel to one another, each segment being adjacent to at least one other segment and to no more than two other segments, adjacent segments being adjoined to one another at substantially planar interfaces. The seven segments include a transparent central segment, transparent first and second exterior segments, and four colored interior segments, two on each side of the central segment. For example, the first, second, third, and fourth interior segments can each have different colors such as red, black, blue, and green. The ball has an anisotropy for providing an electrical dipole moment, the electrical dipole moment rendering the ball electrically responsive such that when the ball is rotatably disposed in an electric field while the electrical dipole moment of the ball is provided, the ball tends to rotate to an orientation in which the electrical dipole moment aligns with the field. Also disclosed are: an apparatus made up of a substrate in which are disposed the aforementioned balls, together with electrodes to facilitate a rotation of balls rotatably disposed therein; and a method for using this apparatus.

INCORPORATION BY REFERENCE

The following U.S. patents are fully incorporated herein by reference:U.S. Pat. No. 4,126,854, (Sheridon, "Twisting Ball Panel Display"); U.S.Pat. No. 4,143,103 (Sheridon, "Method of Making a Twisting Ball PanelDisplay"); U.S. Pat. No. 5,075,186 (Sheridon, "Image-Wise AdhesionLayers for Printing"); U.S. Pat. No. 5,262,098 (Crowley et al., "Methodand Apparatus for Fabricating Bichromal Balls for a Twisting BallDisplay"); U.S. Pat. No. 5,344,594 (Sheridon, "Method for theFabrication of Multicolored Balls for a Twisting Ball Display"); andU.S. Pat. No. 5,389,945 (Sheridon, "Writing System Including Paper-LikeDigitally Addressed Media and Addressing Device Therefor").

RELATED PATENT APPLICATIONS

The following copending, coassigned U.S. patent applications are relatedto this case:

U.S. patent application No. 08/572,779 (Attorney Docket No. D/95115),entitled "POLYCHROMAL SEGMENTED BALLS FOR A TWISTING BALL DISPLAY";

U.S. patent application No. 08/572,778 (Attorney Docket No. D/95115Q1),entitled "APPLICATIONS OF A TRANSMISSIVE TWISTING BALL DISPLAY";

U.S. patent application No. 08/572,819 (Attorney Docket No. D/95115Q2),entitled "CANTED ELECTRIC FIELDS FOR ADDRESSING A TWISTING BALLDISPLAY";

U.S. patent application No. 08/572,927 (Attorney Docket No. D/95115Q3),entitled "HIGHLIGHT COLOR TWISTING BALL DISPLAY";

U.S. patent application No. 08/572,820 (Attorney Docket No. D/95116),entitled "ADDITIVE COLOR TRANSMISSIVE TWISTING BALL DISPLAY";

U.S. patent application No. 08/572,780 (Attorney Docket No. D/95116Q1),entitled "SUBTRACTIVE COLOR TWISTING BALL DISPLAY";

U.S. patent application No. 08/572,775 (Attorney Docket No. D/95116Q2),entitled "MULTITHRESHOLD ADDRESSING OF A TWISTING BALL DISPLAY";

U.S. patent application No. 08/572,777 (Attorney Docket No. D/95116Q3),entitled "FABRICATION OF A TWISTING BALL DISPLAY HAVING TWO OR MOREDIFFERENT KINDS OF BALLS"; and

U.S. patent application No. 08/573,922 (Attorney Docket No. D/95271),entitled "ADDITIVE COLOR TRISTATE LIGHT VALVE TWISTING BALL DISPLAY."

BACKGROUND OF THE INVENTION

The present invention relates to visual displays, and more particularlyto addressable, reusable, paper-like visual displays, and to gyricon ortwisting-ball displays.

Since ancient times, paper has been a preferred medium for thepresentation and display of text and images. The advantages of paper asa display medium are evident. For example, it is lightweight, thin,portable, flexible, foldable, high-contrast, low-cost, relativelypermanent, and readily configured into a myriad of shapes. It canmaintain its displayed image without using any electricity. Paper can beread in ambient light and can be written or marked upon with a pen,pencil, paintbrush, or any number of other implements, including acomputer printer.

Unfortunately, paper is not well suited for real-time display purposes.Real-time imagery from computer, video, or other sources cannot bedisplayed directly with paper, but must be displayed by other means,such as by a cathode-ray tube (CRT) display or a liquid-crystal display(LCD). Typically, real-time display media lack many of the desirablequalities of paper, such as physical flexibility and stable retention ofthe displayed image in the absence of an electric power source.

Attempts have been made to combine the desirable qualities of paper withthose of real-time display media in order to create something thatoffers the best of both worlds. That something can be called electricpaper.

Like ordinary paper, electric paper preferably can be written anderased, can be read in ambient light, and can retain imposed informationin the absence of an electric field or other external retaining force.Also like ordinary paper, electric paper preferably can be made in theform of a lightweight, flexible, durable sheet that can be folded orrolled into tubular form about any axis and conveniently placed into ashirt or coat pocket, and then later retrieved, re-straightened, andread substantially without loss of information. Yet unlike ordinarypaper, electric paper preferably can be used to display full-motion andother real-time imagery as well as still images and text. Thus it isadaptable for use in a computer system display screen or a television.

The gyricon, also called the twisting-ball display, rotary ball display,particle display, dipolar particle light valve, etc., offers atechnology for making a form of electric paper. Briefly, a gyricon is anaddressable display made up of a multiplicity of optically anisotropicballs, each of which can be selectively rotated to present a desiredface to an observer. For example, a gyricon can incorporate balls eachhaving two distinct hemispheres, one black and the other white, witheach hemisphere having a distinct electrical characteristic (e.g., zetapotential with respect to a dielectric fluid) so that the balls areelectrically as well as optically anisotropic. The black-and-white ballsare embedded in a sheet of optically transparent material, such as anelastomer layer, that contains a multiplicity of spheroidal cavities andis permeated by a transparent dielectric fluid, such as a plasticizer.The fluid-filled cavities accomodate the balls, one ball per cavity, soas to prevent the balls from migrating within the sheet. A ball can beselectively rotated within its respective fluid-filled cavity, forexample by application of an electric field, so as to present either theblack or the white hemisphere to an observer viewing the surface of thesheet. Thus, by application of an electric field addressable in twodimensions (as by a matrix addressing scheme), the black and white sidesof the balls can be caused to appear as the image elements (e.g., pixelsor subpixels) of a displayed image.

The gyricon is described further in the patents incorporated byreference hereinabove. In particular, U.S. Pat. No. 5,389,945 (Sheridon,"Writing System Including Paper-Like Digitally Addressed Media andAddressing Device Therefor") shows that gyricon displays can be madethat have many of the desirable qualities of paper, such as flexibilityand stable retention of a displayed image in the absence of power, notfound in CRTs, LCDs, or other conventional display media. Gyricondisplays can also be made that are not paper-like, for example, in theform of rigid display screens for flat-panel displays.

Although the gyricon represents an important step toward the goal ofelectric paper, there is still a long way to go. For example, a gyriconconstructed of black-and-white balls cannot provide a multicolor image.As another example, a gyricon designed to operate in ambient reflectedlight cannot provide a projective or transmissive display. What isneeded is an advanced gyricon technology that can provide a more fullrange of display capabilities while preserving paper-like advantages.

GOODRICH (U.S. Pat. No. 4,261,653, "Light Valve Including DipolarParticle Construction and Method of Manufacture") discloses a lightvalve based on a spherical ball that can be rotated between a firstorientation and a second orientation through the application ofoscillating electric fields of two different frequencies. Goodrich'sspherical ball is made up of a light-absorptive or lightreflectivecentral segment surrounded by transparent intermediate and outersegments. In the first orientation of the ball, the central segment istransverse to the direction of incident light and so blocks the passageof light. In the second orientation of the ball, the central segment isaligned with the direction of incident light and so admits the passageof light, which passes through the transparent portions of the ball.Rotation between the first and second orientations is accomplished bychanging the frequency of an applied oscillating electric field from ahigh frequency (e.g., 10,000 Hz) to a low frequency (e.g., 100 Hz), andtaking advantage of the frequency-dependent dielectric characteristicsof the intermediate segments and the frequency-insensitive dielectriccharacteristics of the outer segments. When the frequency of the appliedfield is high, the dielectric constant of the intermediate segmentsbecomes less than that of the outer segments, and the induced charge inthe intermediate segments causes the ball to orient in the firstorientation. When the frequency of the applied field is low, thedielectric constant of the intermediate segments becomes greater thanthat of the outer segments, and the induced charge in the intermediatesegments causes the ball to orient in the second orientation.

Goodrich's frequency-dependent addressing scheme requires specialized,possibly cumbersome addressing electronics and an AC voltage sourcecapable of delivering high frequencies (e.g., RF frequencies).Goodrich's light valve balls (although said by Goodrich to be "dipolar")would not be responsive to a change in orientation of the electric fieldvector of a steady-state, nonoscillating electric field. Thus Goodrich'soverall approach does not appear to be readily adaptable for use withelectric fields produced from a simple DC voltage source withouttransformation to high-frequency AC.

SUMMARY OF THE INVENTION

According to the invention, a spheroidal ball is provided that issuitable for use in a pseudo-four color gyricon (a gyricon that candisplay two fully saturated colors, two partially saturated colors, anda background color, such as white). The ball has a center point and iscomposed of seven segments arrayed substantially parallel to oneanother, each segment being adjacent to at least one other segment andto no more than two other segments. Each segment adjacent to exactly oneother segment is an exterior segment, and each segment adjacent toexactly two other segments is an interior segment. Adjacent segments areadjoined to one another at substantially planar interfaces. The sevensegments include a transparent central segment, transparent first andsecond exterior segments, and four colored interior segments, two oneach side of the central segment. More particularly, the seven segmentsare as follows: (1) a central segment including the center point of theball, the central segment being an interior segment and having athickness, the central segment being transparent; (2) a first exteriorsegment, the first exterior segment being transparent; (3) a secondexterior segment situated opposite the first exterior segment withrespect to the central segment, the second exterior segment beingtransparent; (4) a first interior segment adjacent to the first exteriorsegment, the first interior segment having a thickness less than thecentral segment thickness, the first interior segment having a firstcolor; (5) a second interior segment adjacent to the first interiorsegment and to the central segment, the second interior segment having athickness less than the central segment thickness, the second interiorsegment having a second color; (6) a third interior segment situatedopposite the second interior segment with respect to the centralsegment, the third interior segment having a thickness less than thecentral segment thickness, the third interior segment having a colordifferent from the first color; and (7) a fourth interior segmentadjacent to the third interior segment and situated opposite the firstinterior segment with respect to the central segment, the fourthinterior segment having a thickness less than the central segmentthickness, the fourth interior segment having a color different from thesecond color and different from the color of the third interior segment.For example, the first, second, third, and fourth interior segments caneach have different colors such as red, yellow, blue, and green. Theball has an anisotropy for providing an electrical dipole moment, theelectrical dipole moment rendering the ball electrically responsive suchthat when the ball is rotatably disposed in an electric field while theelectrical dipole moment of the ball is provided, the ball tends torotate to an orientation in which the electrical dipole moment alignswith the field.

In another aspect of the invention, an apparatus is provided that issuitable for a pseudo-four color display. The apparatus includes asubstrate in which spheroidal balls are disposed, and means forproducing an electric field to facilitate a rotation of a ball rotatablydisposed in the substrate. The substrate has a surface. Each of theballs has five observable aspects, each aspect being associated with acolor observable by an observer situated favorably to observe thesubstrate surface. The five observable aspects include a first aspectassociated with a first color, a second aspect associated with acombination of the first color and another color different from thefirst color, a third aspect associated with a combination of a secondcolor and another color different from the second color, a fourth aspectassociated with a color different from the second color, and a fifthaspect associated with a background color, the background color beingany color with which none of the first, second, third, and fourthaspects is associated. Each ball has an anisotropy for providing anelectrical dipole moment, the electrical dipole moment rendering theball electrically responsive such that when the ball is rotatablydisposed in an electric field while the electrical dipole moment of theball is provided, the ball tends to rotate to an orientation in whichthe electrical dipole moment aligns with the field. The electric fieldproduced by the field-producing means facilitates a rotation of a ballrotatably disposed in the substrate so as to present, to the observersituated favorably to observe the substrate surface, any one of fiveobservable aspects of the ball for which the rotation is facilitated,thereby rendering observable to the observer at least one color withwhich the presented aspect is associated.

In still another aspect of the invention, a method is provided for usinga modulating device made up of electrically anisotropic spheroidal ballsdisposed in a substrate having a surface. Each of the balls has fiveobservable aspects, each aspect being associated with a color observableby an observer situated favorably to observe the substrate surface. Theaspects of each ball include a first aspect associated with a fullysaturated first observable color, a second aspect associated with apartially saturated observable color different from the first observablecolor, a third aspect associated with a partially saturated secondobservable color, a fourth aspect associated with a fully saturatedobservable color different from the second observable color, and a fifthaspect provided by a transparent component of the ball. According to themethod, light from a light source is provided incident on the modulatingdevice. An electric field is applied in a vicinity of a ball rotatablydisposed in the substrate, thereby facilitating a rotation of the ballso as to present, to the observer situated favorably to observe thesubstrate surface, any one of five observable aspects of the ball forwhich the rotation is facilitated. At least a portion of the lightincident on the modulating device is modulated with the modulatingdevice, so as to provide a modulated light.

In yet another aspect of the invention, a method is provided for usingan apparatus made up of electrically anisotropic spheroidal ballsrotatably disposed in a substrate having front and rear surfaces. Eachball has a center point and has seven (7) segments arrayed substantiallyparallel to one another, each segment being adjacent to at least oneother segment and to no more than two other segments. Each segmentadjacent to exactly one other segment is an exterior segment, and eachsegment adjacent to exactly two other segments is an interior segment.Adjacent segments are adjoined to one another at substantially planarinterfaces. The seven segments include: (1) a central segment includingthe center point of the ball, the central segment being an interiorsegment and having a thickness, the central segment being transparent;(2) a first exterior segment, the first exterior segment beingtransparent;(3) a second exterior segment situated opposite the firstexterior segment with respect to the central segment, the secondexterior segment being transparent; (4) a first interior segmentadjacent to the first exterior segment, the first interior segmenthaving a thickness less than the central segment thickness, the firstinterior segment having a first color; (5) a second interior segmentadjacent to the first interior segment and to the central segment, thesecond interior segment having a thickness less than the central segmentthickness, the second interior segment having a second color; (6) athird interior segment situated opposite the second interior segmentwith respect to the central segment, the third interior segment having athickness less than the central segment thickness, the third interiorsegment having a color different from the first color; and (7) a fourthinterior segment adjacent to the third interior segment and situatedopposite the first interior segment with respect to the central segment,the fourth interior segment having a thickness less than the centralsegment thickness, the fourth interior segment having a color differentfrom the second color and different from the color of the third interiorsegment. The apparatus has an array of addressable elements, each arrayelement including at least one ball of the plurality. According to themethod, an array element is selected. A preferred direction oforientation is selected for balls of the selected array element, thedirection of orientation forming an angle with a vector normal to aplanar portion of the substrate surface in a vicinity of the selectedarray element, the angle being adjustable over a continuous range ofangles from 0 degrees to 180 degrees. Balls of the selected arrayelement are aligned with the preferred direction of orientation byapplying an electric field in the vicinity of the selected arrayelement, the electric field having an electric field vector orientedparallel to the selected preferred direction, thereby influencing ballsof the selected array element to rotate so as to align with thepreferred direction of orientation, the planar interfaces of the ballsthus aligned being disposed substantially perpendicular to the preferreddirection of orientation.

The invention will be better understood with reference to the drawingsand detailed description below. In the drawings, like reference numeralsindicate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a technique for fabricating bichromal gyricon ballsin the prior art;

FIGS. 2A-2B illustrate bichromal gyricon balls obtained using unequalliquid flow rates;

FIGS. 2C-2D are close-up cross-sectional views of the edge of a disk towhich pigmented plastic liquids are applied at unequal flow rates;

FIGS. 3A-3D illustrates a multiple-disk assembly for fabricatingmultichromal gyricon balls;

FIGS. 4A-4B show side and top views, respectively, of a multisegmented,highlight color gyricon ball;

FIG. 5 illustrates an example of a highlight color gyricon display;

FIG. 6A illustrates an enlarged cross-section of a portion of ahighlight color gyricon display;

FIG. 6B depicts an electrode assembly used to produce an erasing fieldin a highlight color gyricon display;

FIG. 6C is a close-up view of part of a rectangular region of theelectrode assembly of FIG. 6B;

FIG. 6D is an end view of the electrode assembly of FIG. 6B;

FIG. 6E illustrates bus bar headers suitable for producing an erasingfield for a highlight color gyricon display;

FIGS. 6F-6G illustrate an eraser for a highlight color gyricon display;

FIG. 6H illustrates a gyricon display having recessed erasure powerelectrodes, and a writing stylus adapted for use therewith;

FIG. 6I illustrates an example of a highlight color gyricon display withbuilt-in erasure and writing electrodes;

FIG. 7A illustrates a gyricon ball suitable for constructing an overlaytransparency gyricon or a gyricon-based architectural screen;

FIG. 7B illustrates the use of an overlay transparency gyricon;

FIGS. 7C-7D show how a light-transmissive gyricon can be used in aprojection mode;

FIG. 7E illustrates an application of a gyricon in an architecturalscreen;

FIGS. 8A-8C illustrate an electrode configuration that provides a cantedfield for a gyricon;

FIGS. 8D-8E are examples of the effects of a canted field on a gyriconball;

FIG. 8F illustrates an electrode configuration that is an alternative tothe canted field configuration of FIGS. BA-8C;

FIGS. 9A-9C are different views of a seven-segment ball for apseudo-four color gyricon;

FIG. 9D illustrates a pseudo-four color gyricon that has a layer ofbacking material;

FIG. 10A illustrates a three-segment gyricon ball made up of a coloredinterior segment surrounded on either side by transparent exteriorsegments;

FIG. 10B illustrates an elastomer sheet for a full-color RGB(red-green-blue) gyricon;

FIG. 10C shows a subpixel arrangement for a full-color RGB gyricon;

FIG. 11A illustrates an elastomer sheet for a full-color CMY(cyan-magenta-yellow) multilayer gyricon;

FIG. 11B illustrates a pixel in the sheet of FIG. 11A;

FIG. 11C depicts a cross-section of an elastomer sheet for a CMYmultilayer gyricon;

FIG. 11D is an exploded view of a pixel in the sheet of FIG. 11C;

FIG. 11E illustrates an exploded view of a CMY gyricon having separateaddressing hardware per layer;

FIG. 11F illustrates a CMY gyricon having a single set of addressinghardware for all layers;

FIG. 11G is a series of views in which the position of the addressinghardware is changed relative to the gyricon sheet;

FIG. 11H illustrates close-packing of gyricon balls in a CMY gyricon;

FIG. 11J (please note that there is no FIG. 11I) illustrates afull-color CMYK (cyan-magenta-yellow-black) multilayer gyricon;

FIG. 12A illustrates a three-segment bistate light valve gyricon ballfor use in an ambient color RGB display;

FIGS. 12B-12D are views of a bistate light valve gyricon ball as used toreveal or hide an underlying color dot;

FIG. 12E illustrates a four-segment tristate light valve gyricon ballfor use in an ambient color RGB display;

FIG. 12F is an exploded view of an ambient color RGB gyricon;

FIGS. 12G-12H are views of a tristate light valve gyricon ballpartiallly hiding an underlying color dot;

FIG. 12I illustrates an additional lighting mode for use with bistateand tristate light-valve gyricons;

FIG. 12J illustrates an alternative, two-layer embodiment of thetristate light valve gyricon;

FIG. 13 schematically depicts the modulation of light in a generalizedtristate light-valve color display;

FIG. 14A is a series of views showing gyricon balls of different sizesand thresholds in multi-threshold gyricons;

FIGS. 14B-14D are voltage response graphs for different multi-thresholdgyricons;

FIG. 14E is a series of views showing successive stages of addressing ina multilayer canted-field gyricon;

FIG. 14F is a series of views showing color saturations available in amulti-threshold single-layer gyricon;

FIG. 14G is a series of views showing successive stages of addressing ina multilayer gyricon having multi-threshold color saturation controlwithin each layer;

FIG. 15A illustrates a nonfusing xerographic apparatus for gyricon ballplacement;

FIG. 15B is a highly magnified view a toner-and-bead powder mixture foruse in the apparatus of FIG. 15A;

FIG. 15C illustrates a liquid elastomer being dispensed over a partiallycured elastomer in which gyricon balls have been placed; and

FIG. 15D illustrates a silk-screen apparatus for gyricon ball placement.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Multilayer Polychromal Gyricon Structures

Typically, known gyricon displays are made up of bichromal balls thatare black in one hemisphere and white in the other. Other kinds of ballsare also known. For example, U.S. Pat. No. 4,261,653 (Goodrich) shows amultilayer ball, although it is made at least in part from glass and itsuse depends on a cumbersome addressing scheme involving high-frequencyelectric fields. U.S. Pat. No. 5,344,594 (Sheridon), which isincorporated by reference hereinabove, at FIG. 4 indicates how amultistream fabrication method for bichromal balls can be extended foruse in making certain multilayer, multichromal balls. Even so, thepresent invention uses multichromal gyricon balls in new ways and inconjunction with other new techniques, such as canted fields, to providea wealth of new possibilities for gyricon displays.

FIG. 1 illustrates a technique for fabricating bichromal balls in theprior art, using apparatus 1. Pigmented plastic liquids 21, 22 areapplied to opposite sides 11,12 of a spinning disk 10, which rotatesuniformly about shaft 15. Centrifugal force causes liquids 21, 22 toflow toward the periphery of disk 10, where they combine at the edge toform bichromal ligaments 30 that eventually break up as bichromal balls40. When liquids 21, 22 flow with equal rates to the edge of disk 10,the technique produces bichromal balls with equal hemispheres of color.

FIGS. 2A-2B illustrate the balls obtained when the pigmented liquid flowrates in the spinning-disk technique of FIG. 1 are made unequal. In FIG.2A, ball 240 has segments 241, 242 joined at planar interface 243, andin FIG. 2B, ball 260 has segments 261, 262 joined at planar interface263. Thus in both FIGS. 2A and 2B, the balls are made up of unequalspherical segments of pigmented material, with a planar interface. Thisplanar interface is important, as will be seen below. FIGS. 2C-2Dillustrate how ligaments at the edge of a spinning disk can produce theballs of FIGS. 2A-2B. FIGS. 2C-2D show close-up cross-sectional views ofthe edge of a spinning disk 210 to which pigmented plastic liquids 221,222 are applied at unequal rates. In FIG. 2C, black liquid 221 isapplied at a lesser flow rate than white liquid 222. The resultingligament 230 contains a broad white segment 231 and a narrow blacksegment 232 separated by a planar interface 233. Upon breakup, ligament230 produces balls like the ball 240 shown in FIG. 2A. In FIG. 2D, blackliquid 221 is applied at a greater flow rate than white liquid 222. Theresulting ligament 250 contains a narrow white segment 251 and a broadblack segment 252 separated by a planar interface 253. Upon breakup,ligament 250 produces balls like the ball 260 shown in FIG. 2B. Again,the planar interfaces are important, as will be seen below.

The unevenly segmented balls of FIGS. 2A-2B show that the circularligaments produced by the spinning disk, and the balls that derive fromthese ligaments, act as though they are made from slabs, with the widthsof the slabs depending on the feed rates of the pigmented liquids.

A modification of the spinning-disk technique can be used to fabricatemultichromal balls. The modification uses a spinning multiple-diskassembly instead of a single spinning disk. An example is illustrated inFIG. 3A. Assembly 300 has three disks 310, 311, 312 that rotateuniformly about shaft 315. The concave or "dish-shaped" outer disks 310,312 curve or slope toward the flat inner disk 311 at their respectiveperipheries. Other geometries are possible, and the exact geometry for aparticular embodiment can be determined, for example, by hydrodynamicmodeling, as will be appreciated by those of skill in the art.

The three-disk assembly of FIG. 3A can be used to produce multichromalballs having certain useful properties, as will be discussed below. Itwill be appreciated, however, that other assemblies having differentnumbers of disks can also be used in the present invention, with thenumber and configuration of the disks varying according to the kind ofball that is to be produced.

If differently pigmented plastic liquids are introduced to each side ofeach of the three disks 310, 311, 312 in FIG. 3A, flow patterns ofpigmented liquids at the edge of the disks can be obtained that resultin multichromal ligaments that break up to form multichromal balls. FIG.3B illustrates a close-up cross-sectional view of an example of the flowof pigmented plastic liquids at the edge of the three-disk assembly ofFIG. 3A. First and second liquids 321, 322 flow over opposite sides ofdisk 310, whose downward-sloping edge can be seen in the figure. Thirdand fourth liquids 323, 324 flow over opposite sides of disk 311, andfifth and sixth liquids 325, 326 flow over opposite sides of disk 312.The combined flows give rise to ligament 330, which breaks up intomultilayer balls such as the ball 340 illustrated in FIG. 3C (side view)and FIG. 3D (top view).

Ball 340 has six segments corresponding to the six streams of plasticliquid used to make it. Segments 341 and 342 join at planar interface343; segments 344 and 345, at planar interface 346; and segments 347 and348, at planar interface 349. If different pigments are used in thevarious plastic liquids 321, 322, 323, 324, 325, 326, then ball 340 willbe multichromal. In general, a three-disk assembly like the one shown inFIG. 3A can produce gyricon balls having six segments of up to sixdifferent colors.

More generally, a multi-disk assembly with N disks can be used toproduce gyricon balls having up to 2N segments in arbitrary colorcombinations. Black, white, or other color pigments or dyes can be used,alone or in combination, so that segments can be made in virtually anydesired color or shade. Segments can be made clear by using unpigmented,undyed plastic liquid. Different segments can be made to have differentwidths by adjusting the flow rates of the various plastic liquids usedto make the segments, with faster flow rates corresponding to widersegments and slower rates to narrower segments according to thetechniques illustrated previously with reference to FIGS. 2A-2D. Two ormore adjacent segments can be made the same color so that theyeffectively merge to form a single broader segment.

By way of example, any given gyricon ball segment can be: black; white;clear (that is, essentially transparent and without chroma, like wateror ordinary window glass); a transparent color (e.g., transparent red,blue, or green, as for certain additive color applications; transparentcyan, magenta, or yellow, as for certain subtractive colorapplications); an opaque color of any hue, saturation, and luminance;any shade of gray, whether opaque or translucent; and so forth. For easeof reference, "achromatic colors" will be used hereinbelow to refer tocolors essentially lacking in chroma, that is, to black, white, gray,and clear, and "chromatic colors" will be used hereinbelow to refer toother colors, including red, orange, yellow, green, blue, indigo,violet, cyan, magenta, pink, brown, beige, etc.

Highlight Color Gyricon

FIGS. 4A-4B illustrate a gyricon ball 440 having five segments 441, 442,443, 444, 445. FIG. 4A shows a side view of ball 440, and FIG. 4B showsa top view. As can be seen in FIG. 4A, central segment 443 is quite widein relation to the other segments, and segments 442 and 444 are quitethin. The wide central segment 443 can be made by using two smalleradjacent segments of identical pigment (not shown). The thin segments442 and 444 can be made using low flow rates of their respectivepigmented plastic liquids.

If segments 441 and 445 are made transparent (for example, of clearplastic liquid having an optical refractive index closely matching therefractive indices of the sheet material and dielectric fluid thatsurround ball 440), segment 442 is made in a dark color such as black,segment 444 is made in a contrasting color such as red or blue, and thebroad central segment 443 is made white, a gyricon ball results that issuitable for highlight color. A highlight color display typicallyprovides a black-and-white display with the addition of one other color,such as red, blue, yellow, green, or a "custom color" chosen for aparticular application (for example, a special color used for a companylogo), that can be applied to any selected portion or portions of thedisplay in order to call attention to text or other matter there.Although highlight color is known in other contexts, for example incertain laser printers and copiers, highlight color in gyricon displaysis new to the present invention.

The ball of FIGS. 4A-4B can be used to construct a highlight colorgyricon display. For example, the balls can be uniformly dispersed in asheet of elastomer or other substrate material permeated by dielectricfluid. Each ball resides in its own liquid-filled cavity within thesheet. One or more balls can be used per pixel of display; here, forsimplicity of exposition, one ball per pixel is assumed. The black faceof the ball can be turned toward the observer to provide a black pixel,and the red or other highlight color face can be turned toward theobserver to provide a highlight color pixel. The ball can be turned at90 degrees between these two positions to provide a white pixel. (Whenthe ball is turned so as to provide a white pixel, the edges of theblack and highlight color segments can be observed along with the whitecentral segment; however, this does not significantly affect the overallwhite appearance of the pixel if the black and highlight color edges aremade sufficiently thin.)

FIG. 5 illustrates an example of a highlight color gyricon display. Aportion of highlight color gyricon display 500 is shown in enlargedcross section, with balls 540, 550, and 560 shown in detail. Ball 540has five segments 541 (transparent), 542 (highlight color), 543 (white),544 (black), 545 (transparent) and is oriented in a direction indicatedby arrow a. Ball 550 has five segments 551 (transparent), 552 (highlightcolor), 553 (white), 554 (black), 555 (transparent) and is oriented in adirection indicated by arrow b. Ball 560 has five segments 561(transparent), 562 (highlight color), 563 (white), 564 (black), 565(transparent) and is oriented in a direction indicated by arrow c. Anobserver at I sees ball 540 as a highlight color pixel, ball 550 as ablack pixel, and ball 560 as a white pixel.

The balls for the highlight color gyricon display 500 are made withsegments of different zeta potentials, so that the balls can be orientedto any of the three possible orientations by application of suitableelectric fields. For example, the transparent segment in contact withthe highlight color segment can be made to have the highest positivezeta potential in contact with the working fluid of the gyricon (i.e.,the dielectric fluid that permeates the optically transparent sheet ofmaterial in which the balls are embedded), and the transparent segmentin contact with the black segment can be made to have the highestnegative zeta potential. According to this scheme, in FIG. 5 ball 540 ismade with transparent segment 541 having the highest positive zetapotential of any segment of ball 540 and transparent segment 545 havingthe highest negative zeta potential of any segment of ball 540.Similarly, ball 550 is made with transparent segment 551 having thehighest positive zeta potential and transparent segment 555 having thehighest negative zeta potential, and ball 560 is made with transparentsegment 561 having the highest positive zeta potential and transparentsegment 565 having the highest negative zeta potential.

Because the segments of gyricon balls are made with different zetapotentials, the balls are electrically anisotropic. When a suitableelectric field is applied in the vicinity of a ball, the ball will tendto rotate, with its direction of rotation and its final orientationbeing substantially determined by its electrical anisotropy. The ballwill retain its orientation even after the applied field is removed.

Different plastic materials can have different zeta potentials. The twotransparent sections of a highlight color gyricon ball (for example,segments 541 and 545 of ball 540) can be made from two differentplastics having two different zeta potentials. The zeta potentialcharacteristics of the ball and its various segments can thus be shapedthrough deliberate choice of materials, as well as by choice of coloringagents for the nontransparent segments.

Some illustrative examples of plastic materials suitable for makinggyricon ball segments are polyethylene, polyester, carnuba wax, andcastor wax. (Although waxes are not polymerized hydrocarbons, they are,strictly speaking, plastic materials.) Other materials, such as epoxy,can also be suitable. The same or similar materials can be used for bothtransparent and nontransparent ball segments, with suitable coloringagents being added in the case of the nontransparent segments. Fortransparent segments, the materials are preferably chosen to haverefractive indices closely matching that of the plasticizer liquid usedto swell the elastomer sheet.

To orient an individual highlight color ball so that the black or thehighlight color faces the observer, as for balls 550 and 540 in FIG. 5,respectively, a suitable electric field can be applied perpendicular tothe plane of the sheet of material in which the ball is embedded. Toorient a highlight color ball so that its black and highlight colorsegments are perpendicular to the observer and its white central segmentis displayed, as for ball 560 in FIG. 5, an electric field can beapplied parallel to or in the plane of the sheet of material in whichthe ball is embedded. A matrix addressing scheme (controlled, forexample, by computer or by digital video) can be used to cause the fieldto be applied selectively in the vicinity of an individual ball to beaddressed.

An electrical stylus can also be used to address the balls, thusenabling a human user to write directly on the gyricon. For example, theparallel field can initially be applied to the entire sheet, orientingall the balls so that their white central segments are presented to theuser. This effectively erases the electric paper, giving the user ablank sheet on which to write. Thereafter, the user can apply a stylushaving a positive potential and move it across the surface of the sheetto reorient the balls in the vicinity of the stylus tip so that theirblack faces are presented to the user. The user can apply a stylushaving a negative potential and move it across the surface of the sheetto reorient the balls in the vicinity of the stylus tip so that theirhighlight color faces are presented to the user. The user can erase thesheet by applying the parallel field, so that the sheet can thereafterbe reused.

In general, a gyricon display can be made in various sizes and shapes,and using various kinds of materials for the gyricon balls, elastomersheet, and plasticizer fluid. The highlight color display of FIG. 5 isexemplary in this regard. For instance, the highlight color display ofFIG. 5 can be made about the size of an ordinary sheet of paper byusing, for example, an 8.5 by 11 inch sheet of SYLGARD 184 elastomermaterial of 20 mils (thousandths of an inch) thickness with ISOPAR Lplasticizer and highlight color balls of 100 micron diameters withcenter segments 50 microns thick, the top segment of each ball beingmade from carnuba wax material, the bottom from castor wax material, andthe three interior segments from castor wax colored with carbon black,titanium dioxide, and a chromatically colored dye or pigment to provide,respectively, the black, white, and highlight colors of the interiorsegments.

Many different dyes and pigments can be suitable for use as coloringagents to provide chromatic and achromatic colors in gyricon balls andsegments of gyricon balls, depending on the application and on thematerial or materials used in constructing the balls. By way of example,if the balls are made from wax materials, some dyes that can be usedinclude BAKER CHEMICAL Cresyl violet blue, BAKER CHEMICAL Rhodamine 6G,DUPONT Rhodamine B1, DUPONT Spirit Blue NS, DUPONT Victoria Blue B base,ALLIED CHEMICALS losol Blue, EASTMAN Acridine orange, CALCO OIL blue N,and CALCO OIL black; and some pigments that can be used include DUPONTR900 titanium dioxide, FERRO 6331 black pigment, CABOT MOGUL L carbonblack, and CABOT MONARCH 1000 carbon black.

FIGS. 6A-6G illustrate an example of an electrode structure that canprovide a parallel field suitable for erasing a highlight color gyricondisplay. FIG. 6A shows an enlarged cross-section of a portion ofhighlight color gyricon display 600. Electrode assemblies 620, 630 arelocated on either side of an elastomer sheet 610 in which balls 611 areembedded. Top electrode assembly 620, which is closest to an observer atI, is made of an optically transparent conductor with a very highresistivity. Bottom electrode assembly 630, which is on the oppositeside of sheet 610 from an observer at I, also has a very highresistivity. Each of the top and bottom electrode assemblies 620, 630 isdivided into rectangular regions; in particular, top electrode assembly620 includes regions 621 and bottom electrode assembly 630 includesregions 631. The rectangular regions are separated by low-resistivitybus bars; in particular, bus bars 622 separate the regions 621 of topelectrode assembly 620 and bus bars 632 separate the regions 631 ofbottom electode assembly 630. Both electrode assemblies 620, 630 can beconnected to a power supply (not shown).

Addressing electrodes for gyricons, such as electrode assemblies 620,630, can be made by depositing a conductive material on a glass orplastic (e.g., MYLAR) backing or substrate. Typically, the conductivematerial is indium/tin oxide (ITO), which can be applied to glass bysputtering. A tin oxide (NESA glass) coating can also be used. Theelectrodes formed using these materials are optically transparent, andso are well suited for addressing the gyricon while minimallyinterfering with the visibility thereof.

To minimize the electrical power drain caused by passing current throughelectrode assemblies 620, 630, it is preferable to use electrodes havinghigh surface resistivities. However, if resistivity values are too high,switching speeds will be slow. Resistivity values can range up toapproximately 10⁹ ohms per square (that is, ohms per unit of area),depending on the particular application or environment in which display600 is used.

Preferably, top electrode assembly 620 is made so as not to electricallyisolate the balls 611 from an applied addressing field, such as thefield of a writing stylus. Since an electrode is transparent to electricfields that change at a rate faster than the capacitive response time ofthe electrode, this condition can be met by making the regions 621 of asufficiently high resistivity material. For example, an elastomer layer30 mils thick has a capacitance of approximately 3 picofarads per squarecentimeter. If sheet 610 is formed of such an elastomer layer, theresistivity of top electrode assembly 620 can be made approximately 10⁸ohms per square so that a user can write on display 600 with a styluswhose tip moves across the display surface at a velocity ofapproximately 100 centimeters per second or greater. A DC voltage of,for example, 80 volts can be used for this stylus.

FIG. 6B is a view from above top electrode assembly 620, showing theconfiguration of voltages of top electrode assembly 620 for producingthe erasing field; the same configuration is used for bottom electrodeassembly 630. A voltage V is applied across the surface of each of therectangular regions 621, so that a uniform electric field E runningsubstantially parallel to the surface of sheet 610 is created in theregion of sheet 610. Because the resistivity of the elastomer sheet 610is high, as is the resistivity of rectangular regions 621, the powerdraw is low. The polarity of the applied voltage alternates betweenpositive and negative from one rectangular region to the next, as shown,thus minimizing the voltage requirement of the power supply. The busbars 622 are low resistivity electrodes (e.g., 100 ohms per square) thatallow uniform distribution of voltage across the high resistivityoptically transparent surface material of the rectangular regions 621.

Bus bars 622 can be connected to one another and to the power supply byany appropriate technique, for example, using wired connections alongone or more edges of sheet 610, or by printing bus bar headers on aglass or plastic backing in a pattern such as that illustrated in FIG.6E, in which header 687 is held at a positive voltage and header 688 isheld at a negative voltage. The potential difference between neighboringprongs 687a and 688a, for example, is a voltage V. Thus headers 687, 688and their counterparts in the bottom electrode assembly produce analternating uniform electric field in the plane of sheet 610 throughoutan overlap region 689. Preferably, the length and width of overlapregion 689 exceed, respectively, the length and width of sheet 610.

Bus bars 622 can be, for example, evaporated gold or aluminumelectrodes, or screen-printed silver-filled epoxy. If electrodes 620,630 are formed of ITO sputtered on glass, bus bars 622 can also be ITO,sputtered onto the glass in a separate operation involving the use of amask. By using ITO on glass, both the electrodes and the bus bars can bemade substantially transparent, thereby increasing the visibility of theunderlying gyricon sheet.

FIG. 6C shows a close-up view of a portion of one of the rectangularregions 621. The particular region in question, region 621a, is situatedbetween two of the bus bars 622, namely bus bars 622a and 622b. Thevoltage differential between first bus bar 622a and second bus bar 622bis V. The surface material of region 621a provides a high-resistivityelectrical connection between low-resistivity bus bars 622a and 622b.

FIG. 6D shows an end view of electrode assemblies 620, 630 and theirrespective constituent regions 621, 631. (For simplicity, elastomersheet 610 and bus bars 622 are omitted in this view.) As can be seen,the alternating pattern of applied positive and negative voltages isidentical for both of the electrode assemblies 620, 630. This ensuresthat the electric field is uniformly parallel to the surface of sheet610 within any given region of sheet.

To enable erasure, a power supply can be associated with the highlightcolor gyricon display. For example, in FIG. 6A, when a switch (notshown) is closed, the erasure power supply can be connected to the topand bottom electrode assemblies 620, 630, thereby establishing electricfields parallel to the sheet 610. The fields cause balls 611 to beoriented with their white central segments facing an observer at 1, thuserasing the gyricon display 600. The power supply is required onlymomentarily when the display is erased, so that the actual powerrequirements can be quite modest and the power supply can be made small.For example, an electric field of about 5 volts per mil can be used torealign the gyricon balls, so that if the resistivity of electrodes 620,630 is 10⁸ ohms per square and assuming that ball rotation completes inapproximately 3 milliseconds, the energy required to erase display 600can be, for example, 6 milliwatt seconds, which can be readily suppliedwith, for example, ordinary flashlight batteries.

The power requirement for erasure can be met, for example, by exposingtwo electrodes near the edge of display 600 and touching theseelectrodes to two similarly spaced electrodes on the writing stylus(which is assumed to have its own power supply) when erasure is desired.This is illustrated in FIG. 6H. A stylus 695 has electrodes 696a, 696bthat protrude slightly from the end of the stylus opposite the writingtip. These electrodes are spaced so that they line up with similarlyspaced electrodes 697a, 697b connected to display 600. For safety andconvenience, electrodes 697a, 697b can be disposed in a recess 698,preferably located near an edge or corner of the visible surface ofdisplay 600.

FIGS. 6A-6D do not depict display 600 as including electrode assembliesthat can be used for writing (as opposed to erasing) display 600.Writing on display 600 can be accomplished by means of a stylus or otherexternal device. Alternatively or additionally, write electrodes can beincorporated into display 600 together with the erase electrodes, asillustrated in the cross-sectional view of FIG. 6I. Display 600 haselastomer sheet 610 with balls 611 sandwiched between top and bottomerase electrode assemblies 620, 630, as in FIGS. 6A-6D. Additionally, anupper addressable write electrode assembly 626 is situated above toperase electrode assembly 620, and a lower addressable write electrodeassembly 636 is situated below bottom erase electrode assembly 630.Write electrode assemblies 626, 636 preferably have significantly lowerresistivities than do their counterpart erase electrodes. For example,if top and bottom erase electrode assemblies 620, 630 have resistivitiesof 10⁸ ohms per square, then upper and lower write electrode assemblies626, 636 can have resistivities of 10² ohms per square. Both writeelectrode assemblies 626, 636 and erase electrode assemblies 620, 630can be made matrix-addressable, to facilitate writing and erasure ofindividual pixels.

Erase electrode assemblies can be made external to display 600. Forexample, an erase electrode can be mounted in a special device forerasing electric paper. Any external device that applies an electricfield substantially in the plane of elastomer sheet 610 can be used. Anexample of such a device is the electric paper eraser shown in FIGS.6F-6G. A user can erase a display such as display 600 by holding eraser691 against the visible surface of the display and wiping eraser 691back and forth across the display surface in the direction indicated inFIG. 6F by arrows 694. Thus a user of display 600 can erase some or allof the displayed text, image, or other displayed matter by wiping thedisplay with eraser 691 in a manner analogous to that in which a pieceof conventional paper is erased with a rubber eraser, or in which aconventional whiteboard is wiped clean with a whiteboard eraser.

Eraser 691 is shown in cross-section in FIG. 6G. A resistive surface 692is subjected to a potential difference between V1 and V2, resulting inan electric field E in and parallel to the plane of surface 692. Aninsulating housing 693 surrounds resistive surface 692, giving the userof eraser 691 a way to hold the eraser and also providing packaging fora power source and circuitry (not shown) used to produce the voltages V1and V2 at either end of surface 692. Preferably, the electric field inresistive surface 692 penetrates into sheet 610 to sufficient depth, andis sufficiently parallel to the plane of sheet 610, to ensure a cleanand thorough erasure.

Overlay Transparencies

Overlay transparencies superpose printed, graphical, or other visualinformation on a transparent (for example, clear or lightly tinted)background. They can be placed on top of an underlying opaque page suchas a map, a printed text, or a colored background. Commonly in the priorart, overlay transparencies are made of flexible transparent plasticsheets upon which visual information is printed or marked. Such plasticsheets are non-reusable and are not capable of providing real-timedisplay.

A gyricon display suitable for use as an overlay transparency can beconstructed. The display can be made to have the qualities ofreusability, physical flexibility about any axis, suitability toreal-time imagery, and so forth that are characteristic of gyriconbasedelectric paper; however, the display appears transparent rather thanopaque when it is blank. The display can be used in ambient light if adiffuser, such as a sheet of ordinary white paper, is placed behind it.The display is similar in construction to the highlight color display500 of FIG. 5, except for the balls used, as will now be described.

FIG. 7A illustrates a ball suitable for constructing an overlaytransparency gyricon display. Ball 740 is a five-segment ball with abroad transparent central segment 743, two thin pigmented segments 742,744, and two transparent outer segments 741,745. The transparentsegments 741,743, and 745 are chosen to have an optical refractive indexthat closely matches the refractive indices of the elastomer sheet andthe dielectric fluid that permeates the sheet, so that the ball appearstransparent when oriented with its transparent central segment 743facing the observer (that is, in a 90-degree orientation comparable tothe orientation of ball 560 in FIG. 5).

Ball 740 is made with segments of different zeta potentials, so that itcan be oriented to different orientations by application of suitableelectric fields. In particular, segment 741 can be made with the highestpositive zeta potential of any segment in ball 740, and segment 745,with the highest negative zeta potential of any segment in ball 740.

By applying an electric field in or parallel to the plane of the sheetin which the balls are embedded, the balls can be oriented to presenttheir transparent aspect to the observer, and thus the display can beerased. An electrode configuration like the one described for FIGS.6A-6G can be used to effect erasure.

The pigmentation chosen for segments 742, 744 can depend on the intendeduse of the overlay transparency. If, for example, the overlaytransparency is to be used to highlight elements of an underlyingblack-and-white textual document, the thin segment 742 can be made in anopaque color, such as red or yellow, and the other thin segment 744 canbe made in another opaque color, such as blue or green. As anotherexample, if the overlay transparency is to be used with a plain white orother suitably colored background, such as a background of a gray,beige, or other neutral color, the thin segment 742 can be made blackand the other thin segment 744 can be made in a highlight color.

Ball 740 can be fabricated in a manner similar to that used to fabricatethe five-segment ball 440 of FIG. 4A.

FIG. 7B illustrates schematically the use of an overlay transparencygyricon in conjunction with an underlying document, such as a paperdocument. Overlay transparency 750 is placed over document 751 andilluminated by light from light source 752, such as sunlight or ambientlight. The light incident on transparency 750 is modulated, being passedthrough the transparent segments of some balls and absorbed or partiallyreflected by the opaque segments of other balls, according to whichsegment of each ball is presented to the observer at I. Light thatpasses through transparency 750 to reach document 751 can there beabsorbed (e.g., by black text) or reflected (e.g., by a whitebackground). Light reflected from document 751 can then pass backthrough transparent portions of transparency 750 so as to reach theobserver at I.

An overlay transparency gyricon can also be used in a backlit mode, forexample with a collimated light source such as a projector to make ablack-and-white projected image. This is illustrated schematically inFIG. 7C and in a particular example (an overhead projector 759) in FIG.7D. In each of these figures, light source 753 provides a bright,preferably white light that is collimated by a condensing lens 754,modulated by overlay transparency 755, and thereafter projected by aprojection lens 756 onto a view screen 757 so as to form an imageviewable by an observer at I. If overlay transparency. 755 is made up ofballs like ball 740 that have opaque background and highlight colorsegments, it is not especially well-suited for backlit use, inasmuch asthe background and highlight colors, being opaque, cannot bedistinguished from one another in the image projected on screen 757.Nevertheless, this mode of use can be worthwhile in some instances, andis included here for completeness of exposition. (Gyricon devices thatare capable of producing color projected images and are thus bettersuited for use in projective or other backlit modes modes include theadditive and subtractive color gyricons that are described below withreference to FIGS. 1OA-10C and FIGS. 11A-11C, respectively.)

Architectural Screens

Transmissive polychromal-ball gyricon technology can be applied to makecosteffective, electrically actuated privacy and light control screensthat can be used in architectural and interior design applications, forexample, in electronic window shades, electronic Venetian blinds, orelectronic room partition screens. A gyricon suitable for anarchitectural screen application can be made similar in construction tothe highlight color display 500 of FIG. 5, except for the balls used.These can be like the five-segment ball 740 illustrated in FIG. 7A, witha broad transparent central segment 743, two thin pigmented or dyedsegments 742, 744, and two transparent outer segments 741, 745. The ballappears transparent when oriented with its transparent central segment743 facing the observer.

By applying an electric field in or parallel to the plane of the sheetin which the five-segment balls are embedded, the balls can be orientedto present their transparent aspect to an observer, and thus thearchitectural screen can be made to transmit incident light. Similarly,an electric field perpendicular to the sheet can be used to display thepigmented or dyed aspects of the five-segment balls to the observer. Theperpendicular field can be made addressable, for example at lowresolution (for example, for each rectangular louver or shutter elementof an electronic Venetian blind) or at high resolution (for example, foreach pixel, with one or more balls being used per pixel) according tothe particular application.

The pigmentation or dye chosen for ball segments 742, 744 can depend onthe intended use of the architectural screen. For example,light-absorbing, lightreflecting, or light-scattering pigments can beused, or colored pigments or dyes can be used. Moreover, different ballsin a gyricon can be differently pigmented or dyed, and patterns arepossible, so that designs, patterns, or pictures can be imposed on thearchitectural screens. Thus gyricon architectural screens can be builtthat will be almost fully transparent or that will absorb light, reflectlight, or even create changes in decor at the touch of a button, inresponse to a momentary application of low-level electrical power.

Transparent central segment 743 can be clear, but can also be, forexample, tinted, translucent, or "smoke-glass" colored. Again, theparticular choice made for transparent central segment 743 depends onthe intended use of the screen, and different balls in a gyricon can usedifferent kinds or colors of transparent segments.

An example application for the architectural screen is in a "smart"window. The windowpane can be built to incorporate a transmissivegyricon architectural screen, for example by forming the windowpane oftwo layers of glass with an elastomer sheet containing five-segmentballs situated between the layers. Each glass layer is coated with atransparent electrode coating, such as ITO, disposed toward theelastomer sheet. The transparent electrode coatings are used to applyvoltages to the gyricon.

FIG. 7E is a cross-sectional view of such a window. Window 770 includeswindow frame 775 and windowpane 776. Windowpane 776 has outer layers ofglass 771a, 771b. Layer 771a has a transparent electrode coating 772adisposed toward the interior of windowpane 776 and layer 771b has atransparent electrode coating 772b also disposed toward the interior ofwindowpane 776. Between the electrode coatings 772a, 772b is anelastomer sheet 773 permeated with dielectric fluid and containingfive-segment balls 774. The entire windowpane is thus a gyricon. Theelectrodes can be used to apply voltages V1, V2, V3, V4 (here shown atthe corners of windowpane 776, but more generally at the corners of anyaddressable areal region within windowpane 776) that can be used torotate the five-segment balls to any desired orientation, using thevariable-angle or canted electric field technique described in the nextsection with reference to FIGS. 8A-8C.

A gyricon architectural screen can be made that provides adjustable,continuously variable light transmission ranging from substantiallycomplete transmission of incident light to substantially completeocclusion of incident light. This adjustability can be achieved by usingan electrode configuration suitable for causing partial rotation of theballs, so that the balls can be oriented at any angle with respect tothe surface of the gyricon sheet.

Variable-Angle (Canted) Electric Fields

More generally, an electrode configuration that can produce an electricfield at any angle with respect to the surface of the gyricon sheet, andthus provide continuously variable ball orientation, can be used in awide variety of gyricon devices. For example, it can be used withhighlight color balls having transparent central segments to build agray-scale overlay transparency, or with black-and-white opaquebichromal balls to build a gray-scale ambient-light reflective display.The same configuration can also be used to provide both the addressing(perpendicular) and erasure (parallel) fields in a highlight colorgyricon display and, in particular, to provide individually eraseablepixels. (Although the electrode configuration's ability to provide acontinuous range of electric field angles and ball rotations is notfully used in this case, nevertheless its ability to produce both theparallel and perpendicular fields with a single electrode structure canbe advantageous.)

FIGS. 8A-8C illustrate an electrode configuration that provides anactive matrix array so that individual gyricon balls or groups of ballscan be addressed and can be rotated to any desired angle. The electrodeconfiguration can generate an electric field oriented at an arbitraryangle to the surface of the gyricon sheet in the vicinity of any ball orgroup of balls (for example, a group of balls forming a pixel orsubpixel). Hereinafter, this configuration sometimes will be called acanted-field electrode configuration.

FIG. 8A shows a side view of a portion of a gyricon 800 having acanted-field electrode configuration. Electrode assemblies 820, 830 arelocated on either side of an elastomer sheet 810 in which polychromalballs 811 are embedded. Top electrode assembly 820, which is closest toan observer at I, is made of an optically transparent conductor with avery high resistivity. Bottom electrode assembly 830, which is on theopposite side of sheet 810 from an observer at 1, also has a very highresistivity and can also be transparent, depending on the application.Each of the top and bottom electrode assemblies 820, 830 is divided intorectangular regions; in particular, top electrode assembly 820 includesregions 821 and bottom electrode assembly 830 includes regions 831. Therectangular regions are separated by high-resistivity separators; inparticular, separators 824 separate the regions 821 of top electrodeassembly 820 and separators 834 separate the regions 831 of bottomelectode assembly 830. Both electrode assemblies 820, 830 can beconnected to a power supply (not shown). Separators 824, 834 can be, forexample, made of glass or other substrate material.

For some applications, it is preferable that the top electrode assembly820 be made so as not to electrically isolate the polychromal balls 811from an externally applied addressing field, such as the field of awriting stylus. Since an electrode is electrically transparent toelectric fields that change at a rate faster than the capacitiveresponse time of the electrode, this condition can be met by making theregions 821 of a sufficiently high resistivity material.

Within each of the regions 821 are located individually addressable busbars 822, and within each of the regions 831 are located individuallyaddressable bus bars 832. Top electrode bus bars 822 are situatedparallel to and directly above their counterpart bottom electrode busbars 832. The voltage at each individual bus bar can be set using activematrix addressing electronics (not shown) incorporated into gyricon 800or housed separately. (For example, active matrix addressing electronicscan be pressed into contact with sheet 810 in a manner similar to thatin which active matrix addressing electronics are pressed into contactwith a liquid crystal layer in an LCD.) Thus each of the regions 821,831 can be individually addressed and can correspond, for example, to apixel or subpixel of a pixel-addressable display.

As an example, bus bars 822a and 822b are located on either side of topelectrode region 821a, and bus bars 832a and 832b are located on eitherside of bottom electrode region 831a. Bus bar 822a is parallel to anddirectly above bus bar 832a, and bus bar 822b is parallel to anddirectly above bus bar 832b. The voltage at bus bar 822a is V1; at 822b,V2; at 832a , V3; and at 832b, V4. By addressing the bus bars 822a,822b, 832a, 832b and setting the voltages V1, V2, V3, V4 appropriately,as described more fully below with reference to FIG. 8C, electric fieldscan be established in sheet 810 in the vicinity of these bus bars, sothat the polychromal balls 811 within a parallelepiped-shaped portion ofsheet 810 bounded by bus bars 822a, 822b, 832a, 832b can be addressed asan individual display element.

FIG. 8B shows a portion of top electrode assembly 820, viewed fromabove. Separators 824 criss-cross top electrode assembly 820, and pairedbus bars 822 flank each of the regions 821. For example, region 821a isdelimited by separators 824a, 824b, 824c, and 824d. Bus bars 822a, 822bare situated along either side of region 821a, within the perimeterestablished by separators 824a, 824b, 824c, and 824d. The structure ofbottom electrode assembly 830 (not visible in FIG. 8B) is similar tothat of top electrode assembly 820; in particular, bottom electroderegion 831a is situated below top electrode region 821a, and bus bars832a, 832b are situated parallel to and directly below bus bars 822a,822b, respectively.

FIG. 8C shows three examples of electric fields that can be producedwith the canted-field electrode configuration (fringing effects arenegligible and, accordingly, are not shown). In the first example at A,the electric field lines run parallel to the planes of electrodes 820,830, and thus parallel to the surface of sheet 810 (not shown in FIG.8C). In the second example at B, the electric field lines runperpendicular to the planes of electrodes 820, 830 and thusperpendicular to the surface of sheet 810. In the third example at C,the electric field lines run at an angle θ with respect to the planes ofelectrodes 820,830 and thus with respect to the surface of sheet 810.

The different fields illustrated in FIG. 8C can be generated by settingthe voltages on the bus bars 822, 832 appropriately. For example, if inFIG. 8A the voltages V1, V2, V3, V4 at bus bars 822a, 822b, 832a, 832b,respectively are set so that V1=V3 and V2=V4, then electric field lineslike those at A in FIG. 8C are generated in the vicinity of these busbars, that is, electric field lines running parallel to the planes ofelectrodes 820, 830. If in FIG. 8A the voltages V1, V2, V3, V4 at busbars 822a, 822b, 832a, 832b, respectively are set so that V1=V2 andV3=V4, then electric field lines like those at B in FIG. 8C aregenerated in the vicinity of these bus bars, that is, electric fieldlines running perpendicular to the planes of electrodes 820, 830. If inFIG. 8A the voltages V1, V2, V3, V4 at bus bars 822a, 822b, 832a, 832b,respectively are set so that V1>V3 and V2>V4, then electric field lineslike those at C in FIG. 8C are generated in the vicinity of these busbars, that is, electric field lines running at an angle θ with respectto the planes of electrodes 820, 830. The value of the angle θ isdetermined by the particular values of the voltages, and can be changedby adjusting the voltages. Thus a continuous, 360-degree range ofelectric field directions can be generated.

It will be appreciated that application of a canted field to a gyriconball can cause the ball to rotate, for example, through an angle of lessthan 180 degrees. In FIG. 8D, exemplary gyricon ball 891 in substrate890 has its maximum positive zeta potential at a first end 892 and itsmaximum negative zeta potential at a second end 893. Thus ball 891 has adipole moment, here represented by a vector p. In a gyricon of the priorart, dipole moment vector p would, in the absence of an applied electricfield, preferably be oriented either parallel or antiparallel to avector N defining a normal to a surface 895 of the substrate 890 inwhich ball 891 was disposed. Application of an electric field to ball891 would cause ball 891 to rotate, if at all, through a 180-degreeangle, so that upon deactivation of the field, dipole moment vector pwould once again be either parallel or antiparallel to surface normalvector N. Canted fields can likewise accomplish this 180-degreerotation, but they can also do more. According to the invention,application of a canted field oriented neither parallel nor antiparallelto the surface normal vector N causes the ball to rotate through anangle of less than 180 degrees so as to align with the canted field, andto remain there after the field is turned off until such time asanother, differently oriented electric field is applied. For example, asshown in FIG. 8D, application of a canted field having electric fieldvector E at a time to will cause ball 891 to align with the field byrotating through an angle α; removal of the field at a later time t₁ asshown in FIG. 8E leaves the ball's dipole moment vector p at the angle αto surface normal vector N. To summarize, whereas in the prior art, inwhich the application of an electric field served to rotate the dipolemoment vector of a gyricon ball either through an angle of 180 degrees,or not at all, according to the invention, the application of a cantedelectric field can serve to rotate the dipole moment vector of the ballthrough any desired angle.

An alternative approach to the canted-field electrode configuration isillustrated in the exploded view of FIG. 8F. A gyricon 850 has elastomersheet 853 with gyricon balls 861. Sheet 853 is surrounded by twohigh-resistivity erase electrodes 852, 854 that can generate electricfields in or parallel to the plane of sheet 853, in a manner similar tothat previously described with reference to FIGS. 6A-6D. Alow-resistance ground-plane electrode 851 is disposed on the oppositeside of erase electrode 854 from sheet 853. A matrix addressingelectrode assembly 855 is disposed on the opposite side of eraseelectrode 852 from sheet 853. Thin dielectric separator layers 856a,856b separate, respectively, erase electrode 852 from ground plane 851and erase electrode 854 from addressing electrode assembly 855. Thelayers 856a, 856b can be, for example, a deposited polymer or a plasticsheet. Surrounding the electrode configuration are two substrate layers870a, 870b. At least one face of gyricon 850 is optically transparent.For example, if an observer at I is to view gyricon 850, then substratelayer 870b, addressing electrode assembly 855, dielectric separator856b, and erase electrode 854 preferably should all be transparent. Forsome applications, such as the "smart" window application previouslydescribed with reference to FIG. 7E and other architectural screenapplications, all components outside gyricon sheet 853 (that is, theelectrodes 851, 852, 854, 855, both dielectric separators 856a, 856b,and both substrate layers 870a, 870b) can advantageously be madetransparent.

The voltages V1, V2, V3, V4 provided by the erase electrodes 852, 854should be set such that V1=V3 and V2=V4 for an in-plane erasure field,which when applied to balls 861 causes balls 861 to align with theirelectrical dipole moments in the plane of sheet 853. Alternatively, ifother values of V1, V2, V3, and V4 are chosen, a canted field isproduced by which balls 861 can be oriented with their dipole moments atan arbitrary angle to the plane of sheet 853.

The electrode configuration of FIG. 8F provides only a limitedcanted-field capability. This is because the erase electrodes 852, 854do not provide pixels or other addressable image elements, but insteadact on all the balls 861 together. Addressing electrode assembly 855,which does have addressable elements, in conjunction with ground plane851 can only produce electric fields that are perpendicular to the planeof sheet 853, and cannot produce canted fields. Thus the electrodeconfiguration of FIG. 8F is not readily adaptable to a display in which,for example, it is desired to have different canted field angles on aper-pixel or persubpixel basis. Even so, the configuration can be usefulin certain circumstances, as for example in a low-cose, low-resolutionapplication in which it is desired to have a first subset of balls 861of the gyricon to be oriented with their electrical dipole momentspointed upwards with respect to the plane of sheet 853, a second subsetoriented with their dipole moments pointed downwards with respect to theplane of sheet 853, and a third subset oriented with their dipolemoments at a selected cant angle with respect to the plane of sheet 853,the selected angle being the same for all balls of the third subset. Theconfiguration is also useful in conjunction with certain multithresholdgyricons, as will be discussed below with reference to FIGS. 14A-14G.

Pseudo-Four Color Gyricon

A gyricon with a canted-field electrode configuration can be used toprovide a display having four colors plus white (or another suitablebackground color). A multichromal ball suitable for such a display canbe made with seven segments including a transparent central segment,transparent first and second exterior segments, and four coloredinterior segments, two on each side of the central segment. The ball anddisplay will now be described with reference to FIGS. 9A-9D.

FIG. 9A shows a side view of a seven-segment polychromal ball 940. Thebroad central segment 944 and the endmost segments 941, 947 aretransparent (for example, clear). Each of the four thinner segments 942,943, 945, 946 can be a different color; for example, segment 942 can bered, segment 943 can be green, segment 945 can be yellow or black, andsegment 946 can be blue. Many other color combinations are alsopossible. For example, combinations of achromatic and chromatic colorscan be used; two segments can be made the same color (for instance, bothsegments 943 and 945 can be green, or both segments 942 and 946 can bered); and so forth. The fabrication techniques described above can beused to make ball 940; in particular, the broad central segment 944 canbe composed of two thinner transparent segments of like material thateffectively merge to form the broad central segment.

Ball 940 is made with segments of different zeta potentials, so that itcan be oriented to different orientations by application of suitableelectric fields. In particular, segment 941 can be made with the highestpositive zeta potential of any segment in ball 940, and segment 945,with the highest negative zeta potential of any segment in ball 940.

If ball 940 is rotated so that segment 946 faces an observer, theobserver sees the color of segment 946, for example, blue. This isillustrated in FIG. 9B. Similarly, if ball 940 is rotated so thatsegment 942 faces the observer, the observer sees the color of segment942, for example, red. If ball 940 is rotated to an orientation betweenthese two extremes, for example, by using the canted-field electrodeconfiguration to generate an angled electric field as was described withreference to FIG. 8C, the observer sees a combination of two colors.These will be either the colors of segments 942 and 945 (for example,red and yellow) or, as shown in FIG. 9C, the colors of segments 943 and946 (for example, green and blue).

Finally, white can be obtained by using a white background below theball, and turning the ball to the 90-degree position so that the broadcentral segment 944 faces the observer. The background can be provided,for example, by adhesively attaching an opaque white backing to theelastomer sheet on the side away from the observer. An example is shownin FIG. 9D, which illustrates a side view of a portion of a gyricon 900that includes elastomer layer 910, seven-segment balls 911, and a layerof backing material 912 attached to elastomer layer 910. Alternatively,the backing can be omitted or can be made of a transparent material, sothat the gyricon sheet can be used as an overlay transparency, forexample, to be overlaid on a textual document or other opaque orreflective background.

With this arrangement, the resulting display provides good colorsaturation for the colors of segments 942 and 946, and lesser colorsaturation of the colors of segments 943 and 945. Thus it can displayparts of the color gamut that are unobtainable with a two-color display.

Full-Color (RGB) Additive Color Gyricon

A gyricon with a canted-field electrode configuration can be used toprovide a full-color, red-green-blue (RGB) additive color image. FIG.10A illustrates a three-segment gyricon ball suitable for such adisplay. Ball 1040 has two broad transparent (for example, clear) outersegments 1041,1043 and a thin central segment 1042. For an RGB display,central segment 1042 is pigmented or dyed red, blue, or green. Ball 1040is made with segments of different zeta potentials, so that it can beoriented to different orientations by application of suitable electricfields. In particular, segment 1041 can be made with the highestpositive zeta potential of any of the three segments in ball 1040, andsegment 1043, with the highest negative zeta potential of any of thethree segments.

To make a full-color RGB display, a gyricon sheet can be formed of ballslike ball 1040. For a pixel-addressable RGB display, each pixel caninclude a red subpixel, a green subpixel, and a blue subpixel, with eachsubpixel containing one or more balls of its respective color.Preferably, a subpixel contains a large number of balls (for example,nine or more) located near to one another. A canted-field electrodeconfiguration is provided such that each pixel or subpixel can beindividually addressed and the ball or balls within that pixel orsubpixel can be oriented at any angle with respect to the sheet'ssurface.

FIG. 10B illustrates a side view of a portion of an elastomer sheet 1010from an RGB gyricon. Sheet 1010 contains balls such as balls 1040, 1050,and 1060, each of which has two broad transparent outer segments and athin, colored central segment. Ball 1040 is oriented in a directionindicated by arrow a, with its thin central segment 1042 seen edge-on byan observer at I. In this orientation, which can be achieved by applyingan electric field parallel to the surface of sheet 1010 in the vicinityof ball 1040, ball 1040 appears substantially transparent to an observerat I. Ball 1050, which has transparent outer segments 1051, 1053 andcentral segment 1052, is oriented in a direction indicated by arrow b.In this orientation, which can be achieved by applying an electric fieldperpendicular to the surface of sheet 1010 in the vicinity of ball 1050,central segment 1052 is seen face-on so that ball 1050 appears as afully saturated color to an observer at 1. Ball 1060, which hastransparent outer segments 1061, 1063 and central segment 1062, isoriented in a direction indicated by arrow c. In this orientation, whichcan be achieved by applying an electric field at an angle intermediatebetween parallel and perpendicular to the surface of sheet 1010 in thevicinity of ball 1060, central segment 1062 is seen at an angle, so thatball 1060 appears as a partially saturated color to an observer at I.

If, for example, central segments 1042, 1052, and 1062 are colored red,green, and blue, respectively, then the portion of sheet 1010 indicatedas 101a in FIG. 10B can serve as a pixel having one ball of each color;each of balls 1040,1050,1060 provides a subpixel of this pixel. (Inpractice, as indicated above, an RGB gyricon is likely to have manyballs per subpixel. Nevertheless, the one-ball-per-subpixel arrangementillustrated here is also possible, and provides an easily understoodexample for purposes of exposition and discussion.)

The colored central segments of the balls used in an RGB gyricon, suchas balls 1040, 1050,1060, can be either light-reflective (i.e., ofopaque colors) or light-transmissive (i.e., of transparent colors). Ifthe central segments are light-reflective, the RGB gyricon provides areflective display that can be viewed in ambient light. For example, anRGB gyricon having a transparent elastomer layer and balls withlightreflective central segments can be used as an overlay transparency,in a manner similar to that previously described for transparency 750 inFIG. 7B above. If the central segments are light-transmissive and ifother components are suitably transparent, the gyricon provides atransmissive display that can be viewed by being backlit or used inconjunction with a projector, such as an overhead projector, or by beingplaced on a sheet of ordinary white paper or other diffuser.

For example, an RGB gyricon having a transparent elastomer layer andballs with light-transmissive central segments can be used in aprojector in a manner similar to that previously described fortransparency 755 in FIGS. 7C-7D above. However, whereas the projectedimage produced with the highlight color gyricon previously described wasa black-and-white image, here the image projected on screen 757 appearsin full color. This is because the central color segments of the ballsof the gyricon are transmissive rather than opaque.

As another example, an RGB gyricon having a transparent elastomer layerand balls with light-transmissive central segments can be used in amanner similar to that previously described for transparency 750 in FIG.7B above. However, whereas the black and highlight color segments of theballs in the highlight color gyricon previously described absorbed orreflected incident light, here the color segments of the balls withinthe RGB gyricon act as color filters. White light passing through thecolor segments of the balls can be reflected by an underlying whitesheet of paper (such as document 751) and then pass back through thegyricon to the observer at I, where it will appear red, green, or blueas the case may be. Again, this is because the central color segments ofthe balls of the gyricon are transmissive rather than opaque.

As yet another example, in some cases it is useful to provide a displaythat is readable either by transmitted light or by ambient light. Thiscan be done for an RGB gyricon having a transparent elastomer layer andballs with light-transmissive central segments, again by using anoverlay arrangement in a manner similar to that previously described fortransparency 750 in FIG. 7B above, but with the underlying document 751being replaced by a special surface that appears white in reflectedlight, yet is reasonably transmissive to backlight. A suitable materialfor such a surface is so-called opal glass (available from the EDMUNDSCIENTIFIC CO.; said to be "similar to ground glass but one surface isflashed with a milky white `opal` covering to diffuse light evenly,"Edmund Scientific Co. Catalog #14N1, p. 47). With this arrangement, thedisplay appears white both by reflected ambient light and by transmittedbacklight (e.g., projected light as in FIG. 7D) with the balls orientedwith their center segments perpendicular to the plane of the gyricon, soas to reveal the opal glass to the observer. When the balls are orientedwith their center segments parallel to the plane of the gyricon, thedisplay takes on the colors of the colored center segments, both byreflected and by transmitted light.

The transmissive RGB gyricon does not, by itself, provide a black color.Thus in the previous two examples in which the transmissive RGB gyriconis used with a sheet of white paper or with opal glass, the availablecolor gamut ranges from fully saturated colors to white, but does notinclude black. However, if underlying document 751 is a black and whitedocument, such as an ordinary page of black printed text on white paper,then the black of this document can be perceived through thetransparency 750. Thus an application for which the RGB gyricontransparency can be well-suited is as an "electric highlighter" overlayfor black and white documents, the electric paper analog of ahighlighting marker pen. For this application, RGB color capability isbut one possibility, and other colors in addition to or instead of red,green, and blue can be used for the center segments of the balls thatmake up the gyricon. For example, an electric highlighter gyricon can bemade from three-segment gyricon balls that have center segments of atransparent yellow or pink color, similar to the colors of conventionalhighlighting pens. (If the electric highlighter is to be used to providea single highlight color only, then all the balls in the gyricon canhave the same center segment color and the ball-placement techniquesdescribed below are thus not needed for the gyricon's fabrication.)

To control the red, green, and blue colors of an RGB gyriconindependently of one another, it is necessary to be able to rotate ballsof one color without affecting balls of the other two colors. This canbe achieved, for example, by localizing balls of one color together insubpixels, as illustrated in FIG. 10C. A top view of an enlarged portionof elastomer sheet 1010 is shown. Pixel 1070 includes red subpixel 1071,green subpixel 1072, blue subpixel 1073. Each subpixel contains gyriconballs 1074, 1075, 1076 of its respective color only; for example, allthe gyricon balls 1074 in red subpixel 1071 are red. The arrangement ofthe subpixels within each pixel can vary in different embodiments; forexample, as shown in FIG. 10C, the subpixels can be arranged as thevertices of an equilateral triangle.

Techniques for placement of gyricon balls at specified positions withinan elastomer sheet will be described below with reference to FIGS.15A-15D. These techniques can be used, in particular, to position red,green, and blue gyricon balls in any desired pattern of subpixels.

Multilayer Subtractive Color Gyricon

A gyricon with a canted-field electrode configuration can also be usedto provide a full-color, cyan-magenta-yellow (CMY) subtractive colorimage. In subtractive color imaging, unwanted color components arefiltered out of incident light, typically by means of transparent colorfilters or dyes. Here, the gyricon balls, and more particularly theircenter segments, act as color filters.

A three-segment ball like the ball 1040 illustrated in FIG. 10A can beused for a subtractive color CMY gyricon, with central segment 1042being pigmented or dyed a light-transmissive cyan, magenta, or yellow.The gyricon sheet contains three layers, situated one above the other.One layer contains cyan balls (that is, balls whose central segments arecyan); one contains magenta balls; and one contains yellow balls. Withina given layer, a group of one or more balls can serve to provide acomponent color for color subtraction. Preferably, a large number ofballs (for example, nine or more) located near one another are used foreach component color in each pixel. A pixel is made up of a column ofthree color regions situated above one another, one region from each ofthe three layers.

FIG. 11A illustrates a side view of a portion of an elastomer sheet 1110from a CMY gyricon. Sheet 1110 has three layers 1116, 1117, 1118. Ballsin layer 1116, including balls 1140a, 1140b, and 1140c, have centralsegments of a first color, such as yellow; for example, ball 1140a hasyellow central segment 1142a. Balls in layer 1117, including balls1150a, 1150b, and 1150c, have central segments of a second color, suchas magenta; for example, ball 1150a has magenta central segment 1152a.Balls in layer 1118, including balls 1160a, 1160b, and 1160c, havecentral segments of a third color, such as cyan; for example, ball 1160ahas cyan central segment 1162a. Each of the balls 1140a, 1140b, 1140c,11150a, 1150b, 1150c, 1160a, 1160b, and 1160c can be made individuallyaddressable. The orientation directions of these balls are indicated byarrows a, b, c, d, e, f, g, h, and j, respectively.

A pixel is formed by a combination of one or more color regions that canbe seen in superposition by an observer at I. Thus, for example, theballs 1140a, 1150a, and 1160a in the rectangular columnar portion ofsheet 1110 denoted as 1110a together can form a pixel. Similarly, theballs 1140b, 1150b, and 1160b in the rectangular columnar portion ofsheet 1110 denoted as 1110b together can form another pixel, and theballs 1140c, 1150c, and 1160c in the rectangular columnar portion ofsheet 1110 denoted as 1110c together can form still another pixel. (Inpractice, as indicated above, a CMY gyricon is likely to have many ballsfor each component color in each pixel. Nevertheless, theone-ball-per-color arrangement illustrated here is also possible, andprovides an easily understood example for purposes of exposition anddiscussion.)

The balls 1140a, 1150a, and 1160a all are oriented with their centralsegments 1142a, 1152a, 1162a facing an observer at I, so that full colorsaturation obtains for the cyan, magenta, and yellow components.Accordingly, the pixel at 1110a appears black. The balls 1140c, 1150c,and 1160c all are oriented with their central segments edge-on withrespect to an observer at 1, so that all these balls look substantiallytransparent. Accordingly, the pixel at 1110c appears substantiallytransparent. Ball 1140b is oriented with its central segment facing anobserver at I; ball 1150b, with its central segment at a first anglewith respect to an observer at I; and ball 1160b, with its centralsegment at a second angle with respect to an observer at I. Accordingly,the yellow component of the pixel at 1110b appears fully saturated, themagenta component less saturated, and the cyan component still lesssaturated.

FIG. 11B shows a view from above the pixel at 1110b. The central segmentof ball 1140b appears as a circle A. The central segment of ball 1150bappears as a first ellipse B superposed on the circle A. The centralsegment of ball 1160b appears as a second, narrower ellipse C superposedon the first ellipse B. Thus in the narrow ellipse C, all three colorcomponents (yellow, magenta, cyan) are superposed.

A CMY gyricon can be fabricated either from three separate elastomersheets (one for each color of balls) laid down on top of one another, orfrom a single sheet in which successive layers of different coloredballs are laid down. In either case, each component color region in eachpixel is preferably made up of a large number of balls, and thereforethe balls of one sheet or layer need not be aligned with those of anyother sheet or layer. This is illustrated in the cross-sectional view ofFIG. 11C, which shows a portion of an elastomer sheet 1170 for a CMYgyricon. Sheet 1170 has a layer 1171 of cyan balls 1174 (that is, balls1174 have cyan center segments), a layer 1172 of magenta balls 1175, anda layer 1173 of yellow balls 1176. A pixel 1177 visible to an observerat 1 includes a column-shaped portion of sheet 1170. FIG. 11D shows anexploded view of pixel 1177 separated from the remainder of sheet 1170.(If a CMY gyricon is constructed so that each pixel contains only oneball of each color, as shown in FIGS. 11A-11B, balls in the differentlayers preferably should be aligned so as to facilitate proper colorsubtraction.)

Preferably each of the three layers of a CMY gyricon can be addressedseparately from the other two layers. One way to accomplish this is toprovide a separate addressing electrode for each gyricon layer, asillustrated in the exploded view of FIG. 11E. Gyricon 1180 has a layer1181 of cyan balls, a layer 1182 of magenta balls, and a layer 1183 ofyellow balls. On either side of each layer is disposed a transparentaddressing electrode in the form of a pixel array, so that a differentcanted field can be applied at each pixel position in each layer.Electrodes 1184a, 1184b are disposed on either side of cyan layer 1181.Electrodes 1185a, 1185b are disposed on either side of magenta layer1182. Electrodes 1186a, 1186b are disposed on either side of yellowlayer 1183. Electrodes of successive layers are shielded from oneanother by transparent conductors 1187a, 1187b, 1187c disposed betweenthe electrodes of one layer and the next.

A CMY gyricon having a separate electrode per layer, as shown in FIG.11E, can be cumbersome and expensive to produce. Also, the many layersof electrodes and conductive shielding can cut down on the amount oflight that passes through the gyricon, so that the gyricon image is lessbright than it would otherwise be. Thus it can be preferable in somecircumstances to address all layers of a CMY gyricon with a singleelectrode assembly. For example, in FIG. 11F, sheet 1188 of gyricon 1137is a three-layer elastomer sheet, similar in construction to sheet 1170of FIG. 11C. Electrodes 1189a, 1189b are disposed on either side ofsheet 1188. It is desired to use the electrodes 1189a, 1189b to causerotations of balls of any or all of the three layers of sheet 1188.

A single set of electrodes (as depicted in FIG. 11F) suffices to controlan entire CMY gyricon if balls in different layers of the gyricon havedifferent rotational thresholds. For example, in FIG. 11A, if balls inthe top layer 1116 will begin to rotate only upon application of astrong electric field E1, balls in the middle layer 1117 uponapplication of an intermediate electric field E2, and balls in thebottom layer 1118 upon application of a weak electric field E3, then asingle set of electrodes can be used to address balls of all threecolors in a pixel (for example, to address all of balls 1140a, 1150a,1160a of the pixel at 1110a). As another example, if in FIG. 11C, ballsin top layer 1171 will begin to rotate only upon application of a strongelectric field E1, balls in the middle layer 1172 upon application of anintermediate electric field E2, and balls in the bottom layer 1173 uponapplication of a weak electric field E3, then a single set of electrodescan be used to address all three color components of pixel 1177. In bothexamples, application of the strong electric field E1 will cause ballsin all three layers to rotate. Application of the intermediate field E2will cause only those balls in the middle and bottom layers to rotate.Application of the weak field E3 causes only those in the bottom layerto rotate. Thus the balls in all three layers can be rotated as desiredby successive application of strong, intermediate, and weak addressingfields. This multipass, multithreshold addressing technique is furtherdescribed below with reference to FIGS. 14A-14G.

A pixel-addressable subtractive color gyricon display can be constructedwithout any need for precision alignment of the addressing electrodeswith respect to the gyricon sheet. Instead, pixels will be formedwherever the electrodes happen to be. This is an advantage of thesubtractive color gyricon over the additive color gyricon previouslydescribed with reference to FIGS. 1OA-10C. In the RGB gyricon subpixelarrangement as shown in FIG. 10C, for example, each of the subpixels1071, 1072, 1073 must be aligned properly with the subpixel arrayelements of the addressing electrodes to ensure that an applied electricfield causes one and only one subpixel of the appropriate color to beaddressed. A misalignment between the array elements of the addressingelectrode and the array elements of the gyricon sheet can cause portionsof multiple subpixels of the sheet to be addressed by a single arrayelement of the electrode, and can cause color errors, such assubstitution of red subpixels for green and blue for red, in theproduced image. With the subtractive color gyricon of FIGS. 11A-11D,addressed with a single electrode assembly as in FIG. 11F, none of theseproblems can arise. Instead, the addressing electrodes 1189a, 1189b needonly be aligned to each other, and can be placed anywhere with respectto the gyricon sheet 1188. If a multiple-electrode assembly is used asin FIG. 11 E, the electrodes for the different layers must be alignedwith one another, but again, there is no need to align the layers ofelastomer to the electrodes, or to align the layers to each other. Forexample, it is unnecessary to align cyan layer 1181 with its electrodes1184a, 1184b, or to align cyan layer 1181 with magenta layer 1182 oryellow layer 1183.

The series of views in FIG. 11G further illustrates these ideas. In thefirst view of FIG. 11G, pixel array addressing electrodes 1191a, 1191bare placed on either side of a rectangular multilayer gyricon sheet1190, oriented parallel to the boundaries of the rectangle of sheet1190. Pixels, such as pixel 1192, are formed in rectangular columnarregions of sheet 1190. The locations of pixel 1192 and other pixels withrespect to the boundaries of rectangular sheet 1190 is not predefined.Rather, the placement of electrodes 1191a, 1191b with respect to sheet1190 defines where the pixels will be. If the electrodes are placeddifferently with respect to the sheet, the pixels will end up somewhereelse in the sheet. For example, the second view of FIG. 11G shows whathappens if the addressing electrodes 1191a, 1191b are removed fromcontact with sheet 1190 and thereafter are replaced such that they arenow oriented at an angle β to the boundaries of the rectangle of sheet1190. The pixel array is now skewed with respect to the boundaries ofthe rectangle. For example, the counterpart to pixel 1192 of thefirstview of FIG. 11G is pixel 1192' in the second view of FIG. 11G.

In contrast with the additive color gyricon, there are no subpixels ofdifferent colors in the subtractive color gyricon. Rather, the entirefield of each pixel is filled with all the component colors, superposedon one another. This can improve the richness and accuracy of theresulting color image over what is obtainable with additive colortechniques.

To ensure the highest levels of color saturation and overall imagequality in a subtractive color gyricon, each component color layer ofeach pixel can present a complete ball fill to the observer. Forexample, within each color layer, there can be two or more layers ofballs, stacked one on another as illustrated in FIG. 11H. Gyricon sheet1133 includes layers 1134,1135,1136 that contain three-segment ballshaving cyan, magenta, and yellow center segments respectively. The ballsin layer 1134 are themselves arranged in three close-packed layers1134a, 1134b, 1134c. The balls in layer 1135 are arranged in threeclose-packed layers 1135a, 1135b, 1135c. The balls in layer 1136 arearranged in three close-packed layers 1136a, 1136b, 1136c. Thus littleor no light can pass through gyricon sheet 1133 without being colorfiltered when the balls are suitably rotated. To minimize lightscattering and to obtain the highest light efficiencies, it ispreferable for the balls to have the same refractive indicies as theelastomer and the dielectric fluid permeating the elastomer.

The colors of the cyan, magenta, and yellow center segments of the ballsused in the CMY subtractive color gyricon are typically transparentcolors rather than opaque colors. Thus the CMY gyricon can be used witha backlighting source, such as a projector, or with a reflective backing(e.g., a sheet of ordinary white paper or other diffuser) in ambientlight. Typically, bright incident light is preferable, inasmuch as thegyricon acts to filter out incident light in order to produce the colorsof the image. Examples of use are comparable to those previouslydescribed with reference to FIGS. 7B, 7C, and 7D above as adapted forthe RGB gyricon with balls having transparent central segments. The useof the CMY gyricon as in FIGS. 7C-7D provides a full-color projectedimage on screen 757. The use of the CMY gyricon as in FIG. 7B providesan image in which light filtered by the transparent color segments ofthe balls can be reflected from an underlying surface (e.g., document751) back through the gyricon and thence to the observer at I; notethat, unlike the RGB gyricon, light can be effectively absorbed withinthe CMY gyricon if it is filtered through all three component colors. Asubtractive color gyricon need not be limited to cyan, magenta, andyellow component colors. Other colors can be substituted and additionallayers carrying additional colors can be added. In particular, a fullfour-color CMYK (cyan-magenta-yellow-black) gyricon can be constructedby adding a fourth layer to the CMY gyricon. The CMYK gyricon provides acolor capability analogous to the CMYK color scheme typically used infour-color printing.

FIG. 11J illustrates a CMYK subtractive color gyricon. Gyricon 1195 haslayers 1196, 1197, 1198, and 1199 which contribute, respectively, cyan,magenta, yellow, and black component colors to the image. The gyriconballs used in layers 1196, 1197, 1198 are as described previously forlayers 1116, 1117, 1118 in gyricon sheet 1110 of FIG. 11A. The ballsused in the black layer 1199 are also three-segment balls, similar inconstruction to the balls used in the other layers. However, the centersegments are opaque black rather than transparent chromatic colors as inthe other layers. As for the CMY gyricon, the CMYK gyricon can be usedwith backlighting, such as a projector, or with a reflective backing.Examples of use are comparable to those previously described withreference to FIGS. 7B, 7C, and 7D above, as adapted for the CMY gyricon;note with reference to FIG. 7B that the black segments of the balls inlayer 1199 can fully absorb incident light.

A subtractive color gyricon can also have fewer than three componentcolors. For example, a two-layer gyricon based on three-segment gyriconballs with transparent exterior segments can be made that includes afirst layer of balls with black center segments (as in the CMYK display)and a second layer of balls with center segments of a transparent color.Such a gyricon would be useful for providing a display of black plus ahighlight color over a white background or in a backlit mode. Anotherpossibility is a two-layer gyricon that has a first layer of a firsttransparent color and a second layer of its complementary (alsotransparent) color. For example, the first layer can be blue and thesecond one yellow. This gyricon can produce each of its component colorsplus black, which is produced by the subtraction of two complementarycolors. Controlled by a canted-field electrode, the gyricon thusprovides variable saturations of the component colors plus variable grayscale.

Additive Color Gyricon using Bi-state or Tri-state Light Valves

A gyricon in which the balls themselves have no chromatically coloredsegments can be used to provide a full-color, red-green-blue (RGB)display. Two approaches to such a display will be described. In bothapproaches, the balls in the gyricon sheet act as light valves, in thatthey can be used to reveal or obscure color dots to or from an observer.Using a canted-field electrode configuration as previously described,balls can be rotated through a continuous range of angles, thus allowinga continuous range of color saturation. Each of the dots can be red,green, or blue, and can be formed using, for example, an active lightsource, a backlit colored filter or transparency, or a reflectivecolored backing attached to the gyricon sheet and illuminated by ambientlight. Thus the gyricon can be adapted for use in a backlit orprojective mode or in ambient light.

The two approaches differ in the kind of gyricon balls used. In thefirst approach, a layer of three-segment balls that can act as bistate(two-state) light valves is used to reveal or obscure the color dots.The balls can be, for example, balls similar in construction to thoseused in the RGB and CMY gyricons discussed above, but with opaque (e.g.,white or black) central segments instead of transparent chromaticallycolored central segments. In the second approach, a layer offour-segment ball that can act as tristate (three-state) light valves isused to reveal or obscure the color dots. The balls, which will bedescribed, have black, white, and transparent (e.g., clear) segments. Inan alternative embodiment of the second approach, two layers ofthree-segment balls are used in place of the four-segment ball, onelayer in which the balls have black center segments and the other inwhich the balls have white center segments.

The terminology "bistate" and "tristate" light valves is not meant toimply that these light valves are restricted to only two states or threestates as the case may be. Rather, the light valves have two or threebasic states (eigenstates) and a range of intermediate states obtainablewith the canted field electrode configuration. Thus, as will bediscussed momentarily, the bistate light valve has two basic states:fully open, with the center segment oriented perpendicular to thesurface of the gyricon sheet and the color dot maximally revealed; andfully closed, with the center segment oriented parallel to the surfaceand the color dot maximally obscured. Intermediate states, in which thelight valve is partly open or partly closed are also attainable.Similarly, the tristate light valve has three basic states: fully open,with the center segment oriented perpendicular to the surface of thegyricon sheet and the color dot maximally revealed; fully closed/black,with a black center segment facing the surface and oriented parallel tothe surface, and the color dot maximally obscured; and fullyclosed/white, with a white center segment facing the surface andoriented parallel to the surface, and the color dot once again maximallyobscured.

Turning now to the first approach, a three-segment ball such as thatillustrated in FIG. 12A is used. Ball 1235 has two transparent (forexample, clear) end segments 1236,1238 and a central opaque segment1237. Ball 1235 is made with segments of different zeta potentials, sothat it can be oriented to different orientations by application ofsuitable electric fields. In particular, segment 1236 can be made withthe highest positive zeta potential of any of the three segments in ball1235, and segment 1238, with the highest negative zeta potential of anyof the three segments.

Ball 1235 can act as a bistate light valve, as is depicted schematicallyin the examples of FIGS. 12B-12D. In each of these examples, a color dot1239 is situated below ball 1235 and an observer at I is situated aboveball 1235 and opposite to color dot 1239.

A canted-field electrode is used to orient ball 1235. In FIG. 12B, ball1235 is oriented in the direction shown by arrow a, with its opaquecentral segment parallel to the plane of dot 1239, so that opaquecentral segment 1237 completely obscures dot 1239 from the view of theobserver at I. As shown, dot 1239 appears black to the observer at I. InFIG. 12C, ball 1235 is oriented in the direction shown by arrow b, sothat opaque central segment 1237 is transverse to the plane of dot 1239.As shown, substantially all (more precisely, all but a thin band) of dot1239 is seen by the observer at I. In FIG. 12D, ball 1235 is oriented inthe direction shown by arrow c, so that opaque central segment 1237 isat an angle intermediate between parallel and perpendicular to the planeof dot 1239. (The canted-field electrode permits any intermediate angleto be obtained.) Opaque central segment 1237 partially obscures dot 1239from the view of the observer at I.

A full-color RGB display can be built with balls like ball 1235, forexample by placing a gyricon sheet formed of such balls in front of abackground transparency or backing material patterned with transparentred, green, and blue color filters (for a backlit display), or in frontof a background surface or backing material patterned with reflectivered, green, and blue dots (for an ambient light display). Each pixel ofthe display includes a red, a green, and a blue dot, with each of thesecolor dots corresponding to a subpixel. Each color dot is associatedwith its own independently addressable ball or, preferably, set of manyballs that act as the light valve for that dot. Thus the colorsaturation of a subpixel can be controlled by adjusting the angle of theball or balls over the dot to reveal or obscure more or less of thecolor dot. If many balls are used per subpixel, color saturation canalso be controlled without canted fields by turning a greater or lessernumber of the balls parallel or transverse to the plane of the colordots, using multithreshold techniques as described below with referenceto FIGS. 14A-14G. The arrangement of the subpixels within each pixel canvary in different embodiments; for example, the subpixels can bearranged as the vertices of an equilateral triangle.

Examples of use of the bistate light-valve RGB gyricon in backlit andprojective modes are comparable to those previously described withreference to FIGS. 7B, 7C, and 7D. Used in projective mode as in FIGS.7C-7D, the gyricon provides a full-color RGB projected image on screen757. Used in overlay mode as in FIG. 7B, the gyricon provides atransparent full-color overlay for the underlying document 751. Inoverlay mode, light incident on bistate light-valve gyricon 750 can beabsorbed by, or reflected from, the center segments of the balls whenthe balls are oriented with their center segments parallel to the planeof the gyricon. When the balls are oriented with their center segmentstransverse to the plane of the gyricon, light is transmitted through thetransparent segments of the balls, filtered by the transparent colorfilters, and reflected from white portions of the underlying document751 back through the transparent color filters and the transparent ballsegments of gyricon 750 to the observer at I.

An example of use of the bistate light-valve RGB gyricon as aself-contained (rather than overlay) ambient light display is alsocomparable to FIG. 7B, except that the patterned backing material of thegyricon itself takes the place of underlying document 751. Ambient lightincident on the gyricon can be reflected by or absorbed by the opaquecenter segments of the balls, or can pass through the transparentsegments of the balls to be reflected by the color dots of the patternedbacking material and back through the gyricon sheet to the observer atI.

The first, bistate light-valve approach is well suited for a backlitdisplay. It can be less suitable for a self-contained ambient lightdisplay because of light loss due to scattering, which can lead to a dimand desaturated or "washed-out" display appearance. For example, if theopaque center segments of the balls are black, then "white" is achievedby turning all three subpixels on at once. This "white" can appear grayas compared to the white offered by traditional reflective media, suchas ordinary paper.

The second, tristate light-valve approach addresses this concern. Thetristate light valves have black, white, and transparent states, withthe underlying color dot being visible through the transparent portionsof the gyricon balls. The availability of both white and black inconjunction with red, green, and blue provides an enhanced color gamutover the first, bistate light-valve approach, and makes the secondapproach particularly well-suited for use in ambient light displays.

FIG. 12E illustrates a four-segment gyricon ball suitable for an ambientlight RGB display according to the second approach. Ball 1240 has twobroad transparent (for example, clear) outer segments 1241, 1244 and twothin central segments 1242, 1243. Segment 1242 is pigmented white andsegment 1243 is pigmented black. Ball 1240 is made with segments ofdifferent zeta potentials, so that it can be oriented to differentorientations by application of suitable electric fields. In particular,segment 1241 can be made with the most positive zeta potential of any ofthe four segments in ball 1240, and segment 1244, with the most negativezeta potential of any of the four segments.

To make an ambient color display, a gyricon sheet formed of balls likeball 1240 can be placed over a reflective backing of red, green, andblue color dots. A canted-field electrode configuration is provided suchthat each ball can be individually addressed and oriented at any anglewith respect to the sheet's surface. Each ball can provide a subpixel ofa pixel-addressable display in which each pixel includes a ball locatedabove a red dot, a ball located above a green dot, and a ball locatedabove a blue dot, similarly to the first approach. However, because theball has both black and white central segments, it can do more than justreveal or obscure the underlying dot. Rather, as the ball rotatesthrough 180 degrees, the light from a subpixel will adjust as follows:black, dark color, saturated color, light color, white. For example,with a red dot, an observer will see a range of colors as follows:black, dark red, red, light red, white. (Preferably, as with the bistatelight-valve approach, many balls are used per subpixel, in particular toobviate the need to align individual balls with individual subpixelsduring manufacture of the gyricon. Nevertheless, theone-ball-per-subpixel arrangement described here is also possible, andprovides an easily understood example for purposes of exposition anddiscussion.)

Accordingly, the second approach can increase the brightness of thedisplay, especially in the part of the color space between fullysaturated color and white. The entire color cube of color space can begenerated. Moreover, a light-colored primary color (for example, lightred) can be generated with a single subpixel.

FIG. 12F illustrates an exploded view of a portion of an ambient colordisplay gyricon 1200 in the second approach. Elastomer sheet 1210 isaffixed to a backing 1290 that has red, green, and blue dots (dots 1291,1292, and 1293, respectively). Embedded in sheet 1210 are four-segmentballs including balls 1240, 1250, 1260, which can act as tristate lightvalves to reveal the dots of backing 1290 or to obscure the dots ofbacking 1290 using either their respective black or white centersegments. Thus, for example, ball 1240, whose orientation direction isindicated by arrow a, presents a substantially transparent aspect to anobserver at I, thus revealing a fully saturated red dot 1291. Ball 1250,whose orientation direction is indicated by arrow b, presents a whiteface to an observer at I and completely occludes the view of underlyinggreen dot 1292. Ball 1260, whose orientation direction is indicated byarrow c, is oriented at an angle to the surface of sheet 1210 and thuspresents a portion of a black face to an observer at 1, while revealinga portion of the blue dot 1293 below.

The view from the standpoint of an observer at I of the subpixel formedby ball 1240 in combination with red dot 1291 is schematically depictedfor two different orientations of ball 1240 in the examples of FIGS. 12Gand 12H. In both figures, red dot 1291 appears as a circle A. In FIG.12G, the thin black segment 1243 of ball 1240 is seen as an ellipse Bsuperposed on circle A. Thus the subpixel has black and red colorcomponents, and so appears dark red. In FIG. 12H, the thin white segment1242 of ball 1240 is seen as an ellipse B superposed on circle A. Thusthe subpixel has white and red color components, and so appears lightred.

Examples of use of the tristate light-valve RGB gyricon in backlit andprojective modes are comparable to those previously described withreference to FIGS. 7B, 7C, and 7D. Used in projective mode as in FIGS.7C-7D, the gyricon provides a full-color RGB projected image on screen757. It should be noted, however, that because the black and whitecenter segments are opaque, they are not distinguishable from oneanother in the projected image. Thus the tristate light-valve gyriconhas no color gamut advantage over the bistate light-valve when used inprojective mode. The color gamut advantage obtains when the gyricon isused in overlay or self-contained ambient light modes as in FIG. 7B, thedescription of FIG. 7B here being modified as previously described forthe bistate light-valve gyricon.

An additional lighting mode will now be described with reference to FIG.121. This mode is particularly interesting for use with the tristatelight-valve gyricon, although it can also be used with the bistatelight-valve gyricon. Display 1280 includes a gyricon 1285 that is madeup of a transparent elastomer layer 1287 of four-segment tristatelight-valve balls (including balls 1287a, 1287b, 1287c) attached to atransparent backing material 1286 patterned with transparent colorfilters (including red subpixel filter 1286a, green subpixel filter1286b, and blue subpixel filter 1286c). Gyricon 1285 is illuminatedsimultaneously from the front (that is, from the side closest to theobserver at I) by ambient light originating at light source 1281, andfrom behind by a uniform diffuse white light, here provided by anelectrically powered light source 1282 in conjunction with a diffuser1283, both of which can be built into display housing 1284 as shown.Such built-in backlighting is appropriate, for example, if display 1280is to be used as a display for a laptop computer or the like.

In operation of display 1280, backlight source 1282 serves to make thecolors observable by the observer at I bright and vivid. For example,balls 1287a are oriented so that light from source 1282 is transmittedthrough diffuser 1283 and thence through red color filter 1286a can passthrough the transparent segments of balls 1287a. The observer at I seesa red subpixel. A small amount of ambient light from source 1281 istransmitted through layer 1287 and red color filter 1286a and reflectedby diffuser 1283 back through filter 1286a and layer 1287 to theobserver at I; but the backlighting provides the most significant lightsource for the subpixel.

Ambient light from light source 1281 serves to illuminate the whitecenter segments of balls in layer 1287, so that they are visible to theobserver at I when the balls are oriented with the white center segmentsdisposed toward the front of gyricon 1280. For example, balls 1287b areoriented so as to hide green color filter 1286b from the observer at I.The white center segments of balls 1287b are visible in ambient light tothe observer at I, who sees a white subpixel. Backlighting from source1282 is blocked by the opaque center segments.

Ambient light from light source 1281 is absorbed by the black centersegments of balls in layer 1287. For example, balls 1287c are orientedso as to hide blue color filter 1286c from the observer at I. The blackcenter segments of balls 1287c absorb ambient light. The observer at Isees a black subpixel. Again, backlighting from source 1282 is blockedby the opaque center segments.

FIG. 12J illustrates an alternative embodiment of the tristate lightvalve gyricon. Here, two layers of three-segment balls are used.Elastomer sheet 1270 has layers 1270a and 1270b, each layer containingbistate light valve balls (similar in construction to ball 1235 of FIG.12A). The balls in layer 1270a have white center segments. The balls inlayer 1270b have black center segments. Backing material 1279 providesthe color dots that are revealed or obscured by the balls so as toprovide colors visible to an observer at I.

The two layers of balls cooperate to provide a tristate light valve, asshown: In region 1271 of sheet 1270, balls 1271a in upper layer 1270aand balls 1271b in lower layer 1270b are rotated so that their centersegments are transverse to the plane of backing material 1279, therebyrevealing the underlying dot 1271c to the observer at I. In region 1272of sheet 1270, balls 1272a in upper layer 1270a are rotated so thattheir white center segments are transverse to the plane of backingmaterial 1279, and balls 1271b in lower layer 1270b are rotated so thattheir black center segments are parallel to the plane of backingmaterial 1279, obscuring the underlying dot 1272c and presenting a blackappearance to the observer at I. In region 1273 of sheet 1270, balls1273a in upper layer 1270a are rotated so that their white centersegments are parallel to the plane of backing material 1279, obscuringthe underlying dot 1273c and presenting a white appearance to theobserver at I.

Balls in the two layers 1270a, 1270b can be separately addressed byusing multithreshold, multipass addressing as per FIGS. 14A-14G below.If many balls are used per subpixel, there is no need to align the upperand lower layers with one another during manufacture.

The bistate and tristate light valve approaches both offer certainadvantages over the RGB gyricon described previously with reference toFIGS. 10A-10C. Notably, there is no need to place different kinds ofballs in different places within the elastomer sheet. Instead, the sameballs are used throughout the gyricon, and the RGB subpixel regions of areflective backing material, for example, can be printed on the materialusing conventional printing or color xerographic techniques.Furthermore, if many balls are used per subpixel, the elastomer sheetneed not be precisely aligned with the backing material duringmanufacture. (However, it is necessary to align the subpixels of thebacking material with the subpixels of the addressing electrodes.)

It will be appreciated that the bistate and tristate light valveapproaches can be used with color displays other than RGB displays. Forexample, dots of additional colors can be added. As another example, ahighlight color display can be made using a backing material of a singleuniform highlight color, such as red or yellow, in place of thedot-patterned backing material previously described. Such a display,used for example with a canted-field electrode, can provide a full rangeof the highlight color, varying from white through light color to fullysaturated color to dark color to black, and, like the CMY gyricondescribed earlier, does not require precise alignment between thegyricon sheet and the addressing electrode assembly. Instead, the pixelsof this highlight color display are defined by the relative placement ofthe gyricon sheet and the electrodes, in a manner similar to thatpreviously described with reference to FIG. 11G above. Still further,for use in ambient light, the center segments of the balls (for example)need not be black or white. For example, a highlight color overlaytransparency for use with a white background (e.g., ordinary whitepaper) can be made by providing a transparent elastomer and tristateballs having black and highlight color center segments (e.g., black andred segments). Many other variations will be apparent to persons skilledin the art.

The operational principles of the tristate light valve RGB display canbe generalized beyond the embodiments described here. Notably, it is notnecessary that the tristate light valves be gyricon-based. As othertristate light valves and devices or technologies suitable for producingsuch valves are developed, they can be adapted for use in color displaysin which color sources, such as RGB or other chromatic color sources,are revealed or obscured by tristate light valves according to theprinciples that have been described above. This is illustratedschematically for a single pixel in FIG. 13. A light source 1300provides a colored light, such as a chromatically colored light, for thepixel. A selector 1305 selects between black and white mixture colors,and a mixture control 1307 selects the proportions in which the coloredlight from light source 1300 and the mixture color (black or white)selected with selector 1305 are to be mixed. The colored light is mixedwith the selected black or white mixture color in a color mixer 1310,and the resulting color mixture is provided to output 1315.

Multithreshold and Multipass Addressing Techniques The CMY and CMYKsubtractive color gyricons and the two-layer tristate valve gyricondescribed above require separate addressability for different balls indifferent layers. It can be advantageous to provide this separateaddressability without using a separate addressing electrode for eachlayer. More generally, there are various circumstances in which it canbe beneficial to use a single electrode to address separately differentgroups of different kinds of balls within a gyricon sheet. For example,one way to achieve variable color saturation in an RGB gyricon based onthree-segment balls (as in FIGS. 10A-10C) or a CMY gyricon (as in FIGS.11A-11C) without using canted fields is to provide a multiplicity ofballs associated with each color in each subpixel. This multithresholdtechnique for controlling color presence will be described more fullybelow with reference to FIG. 14F.

Selective rotation of different kinds of gyricon balls disposed in thesame vicinity within the elastomer sheet of the gyricon can beaccomplished if each kind of ball has a distinct rotation threshold,that is, a minimum electric field strength to which it will respond. Forexample, in the two-layer arrangement of FIG. 12J, suppose that theballs in layer 1270a will begin to rotate only upon application of anelectric potential gradient of at least 90 volts per 30 mils ofelastomer thickness, and that the balls gyricon in layer 1270b willbegin to rotate only upon application of an electric potential gradientof at least 80 volts per 30 mils of elastomer thickness. Then if thetotal thickness of sheet 1270 is 30 mils (that is, 15 mils per layer),an 80-volt electric potential difference applied across sheet 1270 willcause balls in layer 1270b to rotate but will not affect balls in layer1270a. A 90-volt potential difference across sheet 1270 will cause ballsin both layers 1270a and 1270b to rotate. For example, the balls inregion 1271 can be oriented as shown, with their center segmentsoriented transversely to the plane of backing material 1279, by a singleapplication of a 90-volt potential difference across both layers ofsheet 1270 in region 1271. The balls in region 1273 can be oriented asshown, with their center segments oriented parallel to the plane ofbacking material 1279, by a single application of a 90-volt potentialdifference in the plane of sheet 1270 in region 1273. The balls inregion 1272 can be oriented as shown, with the center segments of balls1272a in layer 1270a oriented transversely to the plane of backingmaterial 1279 and the center segments of balls 1272b in layer 1270boriented parallel to the plane of backing material 1279, by anapplication of a 90-volt potential difference across sheet 1270 inregion 1272 (which turns the balls to the same orientation as the ballsin region 1271) followed by an application of an 80-volt potentialdifference in the plane of sheet 1270 in region 1272 (which turns balls1272b in layer 1270b so that their center segments are parallel to theplane of backing material 1279 but leaves balls 1272a in layer 1270aunaffected).

Thus it can be seen that for two different kinds of balls, at most twodifferent electric field applications, or "passes," suffice to orientthe balls as desired. In general, if there are N distinct sets of ballsto be addressed, at most N passes are required.

FIGS. 14A-14G illustrate various principles and applications ofmultithreshold, multipass gyricon addressing. FIG. 14A shows one waythat different thresholds can be achieved: namely, by varying the sizeof the balls. Two examples are shown. In example (a) spherical balls ina first layer 1401 of a three-layer gyricon sheet 1400 have a firstradius R₁, spherical balls in a second layer 1402 have a second radiusR₂, and spherical balls in a third layer 1403 have a third radius R₃. Inexample (b) a single layer gyricon sheet 1405 includes balls 1405a,1405b, and 1405c, having first radius R₁, second radius R₂, and thirdradius R₃ respectively. In both examples (a) and (b), R₁ >R₂ >R₃. Otherthings being equal, the amount of torque that must be applied to a ballof radius R₁ in order to cause the ball of radius R₁ to rotate from astanding start will tend to be greater than the amount of torque thatmust be applied to a ball of radius R₂ in order to cause the ball ofradius R₂ to rotate from a standing start. Similarly, other things beingequal, the amount of torque that must be applied to a ball of radius R₂in order to cause the ball of radius R₂ to rotate from a standing startwill tend to be greater than the amount of torque that must be appliedto a ball of radius R₃ in order to cause the ball of radius R₃ to rotatefrom a standing start.

A simple calculation illustrates why. Assuming that the balls are ofconstant mass density and the electrical dipole moment of each ballarises from a separation of positive and negative surface charges insegments at opposite ends of the ball, it follows that balls in layer1401 have greater mass, greater moment of inertia and greater dipolemoment than balls in layer 1402, and balls in layer 1402 have greatermass, moment of inertia, and dipole moment than balls in layer 1403.However, it can be shown that the moment of inertia grows faster withincreasing radius than does the dipole moment, as follows:

(1) Mass M of a uniform solid sphere of radius R and mass density ρ isgiven by M=4/3 ρηR³ ; moment of inertia I of a solid sphere of mass Mand radius R about a central axis is given by I=2/5 MR² ; thus moment ofinertia I increases as the fifth power of the radius R.

(2) Electrical dipole moment p is proportional to quantity of charge qat either end of the ball times the distance D separating the positiveand negative charges, that is, p∝qD; quantity of charge q isproportional to surface charge density σ times surface area over whichthe charge is spread, that is, q∝4ΩσρR² where Ω is the number ofsteradians subtended by the end segments of the ball; charge separationD is proportional to R; thus the electrical dipole moment p of the ballincreases as the third power of the radius R.

(3) Torque of an electric field on a dipole is given by the crossproduct of the electric field vector with the dipole moment vector, thatis, τ=p×E; applied torque equals the product of the moment of inertiatime the angular acceleration of the ball in response to the appliedtorque, that is, τ=Iα; setting these equal gives 1α=p×E.

(4) Therefore, the magnitude of the electric field E that must beapplied to achieve a given angular acceleration α, and thus to overcomea given amount of starting resistance to rotation, varies as the ratioof the moment of inertia I to the dipole moment p, that is, as I÷p=I÷qD.Solving this for E gives E∝(2/5 R²) (4/3 ρηR³)÷(4ΩσηR²)R, or E∝R².

The foregoing calculation is a rough-and-ready estimate, rather than aprecise formula. Nevertheless, it suggests that the greater the radiusof the ball, the stronger the electric field that must be applied tocause rotation.

The calculation also suggests that other parameters, such as the dipolemoment, can be varied to affect rotation threshold. In general, a widevariety of parameters, both of the balls themselves and of the cavitiesin the elastomer in which the balls rotate, can affect the rotationalthresholds of the balls. (It is more precise to speak of "the rotationalthreshold of a particular ball in a particular cavity filled with aparticular dielectric fluid in a particular elastomer," the overallthreshold being a result of a complex interaction between the ball andits environment. Nevertheless, it is convenient to speak of "thethreshold of the ball" as though the rotational threshold wereassociated with the ball alone. That practice is adopted here, it beingunderstood that other factors come into play as well.)

Some of the factors that can affect the strength of the electric fieldthat must be applied to cause rotation of a given ball include: theball's electrical characteristics, such as the electrical dipole momentof the ball and monopole moment, if any, due to the zeta potentials ofthe ball segments which arise when the ball is disposed in dielectricfluid within the substrate; the ball's mechanical characteristics,especially those affecting moment of inertia, such as mass, distributionof mass within the ball, shape of the ball (including deviation of theball from a purely spherical shape to an ellipsoidal or other shape),size, and radius or mean radius, as well as characteristics affectingthe ball's interaction with its spherical cavity, such as the ball'scoefficient of friction and surface roughness; the structure of theball, including the sizes and shapes of any component segments or otherregions within the ball and the disposition of these component segmentsor other regions relative to one another; and the materials that make upthe ball and its segments or regions, including any material(s) used inthe manufacture of any component region(s) within the ball and anymaterial used to coat all or part of the surface of the ball. Additionalfactors that can affect the strength of the electric field that must beapplied to cause rotation of a given ball include: the characteristicsof the cavity in which the ball is situated, such as deviation frompurely spherical shape (including use of piezoelectric fields to affectthis, as disclosed in U.S. Pat. No. 4,126,854, incorporated hereinaboveby reference, at col. 5, lines 16-29) and surface roughness or otherfactors affecting coefficient of friction of the ball against the cavitywall; the material and mechanical characteristics of the elastomer,including the stickiness of the elastomer material; and thecharacteristics of the plasticizer fluid permeating the elastomer andfilling the cavity, including viscosity and dielectric properties. Theforegoing lists of factors are illustrative and by no means exhaustive.

It should also be noted that other things being equal, a thickerelastomer layer requires a higher applied voltage perpendicular to thelayer surface in order to cause rotation of balls of a given threshold.Similarly, for in-plane fields, the larger the width of the substrateregion (e.g., the pixel or subpixel) to which the field is applied, thegreater must be the voltage applied between one side of the pixel andthe other. These observations follow from the definition of the electricfield as the gradient of the electric potential; for a uniform electricfield this reduces to E=V/d, where V is the applied voltage and d is thedistance over which the voltage is applied.

The graph of FIG. 14B illustrates the behavior in response to an appliedvoltage of an ideal gyricon in which the gyricon balls are of threedifferent rotation thresholds.

The graph plots the number of balls that rotate in response toapplication of a given electric field (ordinate) versus the voltage thatmust be applied to a given thickness of elastomer sheeting in order toproduce that field (abscissa). For applied voltages below thresholdpotential φ₃, no balls rotate. For applied voltages greater than orequal to threshold potential φ₃ and less than threshold potential φ₂,balls having the third (lowest) threshold rotate, while other balls areunaffected. For applied voltages greater than or equal to thresholdpotential φ₂ and less than threshold potential φ₁, balls having thethird or second (intermediate) threshold rotate, while balls having afirst (highest) threshold are unaffected. For applied voltages abovethreshold potential φ₁, all the balls rotate.

In a practical gyricon, the ideal graph of FIG. 14B is modified somewhatbecause of statistical variations among the balls. Typically, a givenpopulation of balls having approximately equal physical characteristicswill have a range of thresholds clustered around a mean value, due tominor variations in size, shape, electrical characteristics and so forthfrom one ball to another. Thus the response graph is not likely to bethe series of step functions of FIG. 14B. Instead, for a gyricon havingthree populations of balls, each population having a different averagerotation threshold, the graph will be as shown in FIG. 14C. As thevoltage is increased from zero, no balls rotate until a minimumthreshold potential φ₃ is reached, at which point balls of the third(lowest-threshold) population begin to rotate. As the voltage is furtherincreased over the range between φ₃ and φ₃ +Δ₃, an increasing number ofballs of the third population rotate until, after the voltage exceeds φ₃+Δ₃, all of the balls of the third population will rotate in response tothe applied voltage. If the voltage is further increased to a secondminimum threshold potential φ₂, balls of the second(intermediate-threshold) population begin to rotate, along with all theballs in the third population. As the voltage is further increased overthe range between φ₂ and φ₂ +Δ₂, an increasing number of balls of thesecond population rotate until, after the voltage exceeds φ₂ +Δ₂, all ofthe balls of the third and second populations will rotate in response tothe applied voltage. Further increasing the voltage beyond a thirdminimum threshold potential φ₁ causes some balls of the first(highest-threshold) population to rotate along with all the balls in thethird and second populations. Finally, as the voltage is furtherincreased over the range between φ₁ and φ₁ +Δ₁, an increasing number ofballs of the second population rotate until, after the voltage exceedsφ₁ +Δ₁, all of the balls of all three populations will rotate inresponse to the applied voltage.

When a sharp threshold response is desired (for example, whenpassive-matrix rather than active-matrix addressing electronics are tobe used), the values of Δ₃, Δ₂, and Δ₁ in FIG. 14C should preferably bemade as small as possible. This can be done, for example, by tighteningmanufacturing tolerances for the balls so as to reduce the variance ofany physical characteristics of the balls strongly affecting rotationthreshold (e.g., radius). In any case, the values of Δ₃, Δ₂, and Δ₁should be sufficiently small that the ranges of voltages used foraddressing different sets of balls do not overlap. That is, ifindividual sets of balls are to be separately addressed, theinequalities φ₃ +Δ₃ <φ₂ and φ₂ +Δ₂ <φ₁ must be strictly satisfied.

Alternatively, it can be advantageous in some cases to make the valuesΔ₃, Δ₂, and Δ₁ larger rather than smaller. This is shown in FIG. 14D.The behavior of the balls in response to increasing applied voltage issimilar to that described with reference to FIG. 14C. However, becauseΔ₃, Δ₂, and Δ₁ are larger relative to their respective minimumthresholds φ₃, φ₂, and φ₁) than was the case in FIG. 14C, the slope ofthe graph in the threshold regions is more gentle. If multithresholdingis being used to control color saturation, as will be described withreference to FIG. 14F below, this means that the rate at which eachcolor saturates with increasing applied voltage is more gradual with thewider Δ values of FIG. 14D than would be the case with the narrower Δvalues of FIG. 14C. Thus finer control over color saturation ispossible. Once again, the inequalities φ₃ +Δ₃ <φ₂ and φ₂ +Δ₂ <φ₁ must bestrictly satisfied, and preferably the gaps Y₃₂ and Y₂₁ betweensuccessive Δ ranges should be substantial. For example, if the overallelastomer sheeting thickness is 30 mils (that is, 10 mils per layer fora three-layer CMY gyricon), some possible minimum threshold values areφ₃ =80 volts, φ₂ =90 volts, and φ₁ =100 volts, with Δ₃ =Δ₂ =Δ₁ =5 volts.Consequently the gaps Y₃₂ and Y₂₁ are 5 volts.

A gyricon in which the gyricon balls have multiple rotation thresholdscan be addressed with multiple-pass addressing. The series of views inFIG. 14E depicts successive stages in addressing one pixel of athree-layer CMY gyricon in which all the balls within any given layerhave a common, ideally sharp threshold (i.e., Δ₃ =Δ₂ =Δ₁ =0). The viewsof the series are all side views of a single-pixel region in gyriconsheet 1410. For a thickness T of elastomer, balls in layer 1413 have alowest threshold potential φ₃, balls in layer 1412 have an intermediatethreshold potential φ₂, and balls in layer 1411 have a highest thresholdpotential φ₁. Each layer is to be addressed with a canted field,generated on a per-pixel basis by a canted-field electrode that canprovide voltages V1, V2, V3, V4 at the periphery of the rectangularcolumnar region of sheet 1410 that makes up the pixel, as shown. Thesingle-pixel region of sheet 1410 is assumed to have thickness T andwidth W.

In the first view of FIG. 14E, corresponding to the first addressingpass, the voltages are set such that V1=V3 , V2=V4, and (V3-V2)/W>φ₁ /T.The resulting electric field E1 has a magnitude (V3-V2)/W greater thanthe threshold electric field magnitude ε₁ =φ₁ /T required to causerotations of balls in layer 1411. The field E1 is oriented in thedirection of arrow a. Application of the field E1 causes the balls ofall three layers 1411, 1412, 1413 to align their respective dipolemoments with the applied field. The dipole moment of each ball, whicharises from the zeta potential difference between the end segments ofthe ball in the presence of the dielectric fluid permeating sheet 1410(as indicated in the first view of FIG. 14E by + and - signs in the endsegments) is perpendicular to the plane of the center segment of theball. Thus the center segments of the balls of all three layers arecaused to be oriented parallel to the direction of arrows a' (that is,perpendicular to the planar surfaces 1419a, 1419b of gyricon sheet1410).

In the second view of FIG. 14E, corresponding to the second addressingpass, the voltages are set such that V3>V1, V1=V4, V4>V2, and φ₁/T>(V3-V2)/Y>φ₂ /T, where Y=(T² +W²)^(1/2). The resulting electric fieldE2 has a magnitude of (V3-V2)/Y, which is greater than the thresholdelectric field magnitude ε₂ =φ₂ /T required to cause rotations of ballsin layer 1412. The field E2 is oriented in the direction of arrow b.Application of the field E2 causes the balls of layers 1412 and 1413 toalign their respective dipole moments with the applied field and has noeffect on balls of layer 1411. The center segments of the balls oflayers 1412 and 1413 are caused to be oriented parallel to the directionof arrows b' (that is, at an acute angle with respect to the planarsurfaces 1419a, 1419b of gyricon sheet 1410).

In the third view of FIG. 14E, corresponding to the third addressingpass, the voltages are set such that V1=V2, V3=V4, and φ₂ >V3-V2>φ₃. Theresulting electric field E3 has a magnitude of (V3-V2)/T and is orientedin the direction of arrow c. Application of the field E3 causes theballs of layer 1413 to align their respective dipole moments with theapplied field, which in turn causes the center segments of the balls oflayer 1413 to become oriented parallel to the direction of arrows c'(that is, parallel to the planar surfaces 1419a, 1419b of gyricon sheet1410). The balls in layers 1411 and 1412 are not affected, because theapplied voltage gradient is below their respective thresholds φ₁ /T andφ₂ /T.

Multiple-pass addressing can also be used to address selectively ballsof different rotation thresholds within a single layer of a single-layeror multilayer gyricon. An application of this technique is forcontrolling the color saturation of a chromatic color in an imageelement, the gray scale level for black in an image element, or, ingeneral, the degree to which a color or other optical modulationcharacteristic is observably present in an image element, without theneed for canted fields. For example, one way to achieve variable colorsaturation in an RGB gyricon based on three-segment balls (as in FIGS.10A-10C) without using canted fields is to provide a multiplicity ofballs associated with each color in each subpixel. To get a fullysaturated color, all of the balls in the subpixel are turned with theircenter segments parallel to the surface of the elastomer sheet. To get aminimally saturated color, all of the balls in the subpixel are turnedwith their center segments perpendicular to the surface of the elastomersheet. To get an intermediate color saturation, a subset of the balls inthe subpixel are turned with their center segments parallel to thesurface of the elastomer sheet, while the remaining balls of thesubpixel are turned with their center segments perpendicular to thesheet surface. In other words, the more balls that are turned so thattheir center segments are parallel to the plane of the gyricon sheet,the more saturated the resulting color of the subpixel appears. The sameprinciple can be used to control color saturation without the use ofcanted fields in other color gyricons, such as CMY(K) or bistate ortristate light-valve gyricons. It can also be used to provide gray-scalecapability in, for example, gyricons based on black-and-white bichromalballs of the prior art. The darkness of the gray of a pixel depends onthe percentage of balls in that pixel which have their white and blackhemispheres facing toward the observable surface of the gyricon sheet.

The series of views in FIG. 14F depicts several different degrees ofcolor saturation obtainable in a single-layer gyricon having threedifferent sets of balls disposed within the single layer, each set ofballs having a different rotation threshold, all three sets beingassociated with the same observable color. Each of the gyricon balls isa three-segment ball with transparent end segments and a colored centersegment. For example, if the colored center segments are red, the ballscould be disposed in a single red subpixel of the RGB gyricon previouslydescribed with reference to FIGS. 10A-10C.

The views in FIG. 14F are all side views of a region constituting oneaddressable image element (e.g., subpixel) in gyricon sheet 1420. Forclarity of exposition, a single ball of each threshold is shown,although in practice, preferably a large number of balls of eachthreshold are dispersed uniformly (e.g., randomly) throughout eachpixel. For a thickness T of elastomer, ball 1423 has a lowest thresholdpotential φ₃, ball 1422 has an intermediate threshold potential φ₂, andball 1421 has a highest threshold potential φ₁. Once again, as in FIG.14E, the layer thickness is T and the image element width is W, andideally sharp thresholds are assumed (i.e., Δ₃ =Δ₂ =Δ₁ =0.

In the first view of FIG. 14F, balls 1421, 1422, and 1423 all areoriented with their center segments parallel to the plane of gyriconsheet 1420. An observer at I sees a maximally saturated color. Thisorientation of the balls is obtainable by applying an electric fieldperpendicular to the plane of gyricon with a field strength E>φ₁ /T, orin other words, a voltage difference V across sheet 1420 such that V>φ₁.

In the second view of FIG. 14F, ball 1421 is oriented with its centersegment perpendicular to the plane of gyricon sheet 1420, and balls 1422and 1423 are oriented with their center segments parallel to the planeof gyricon sheet 1420. An observer at I sees a moderately saturatedcolor. This orientation of the balls is obtainable by applying, in afirst pass, an electric field in the plane of gyricon with a fieldstrength E₁ >φ₁ /T (in other words, a voltage difference V such thatV/W>φ₁ /T), and thereafter applying, in a second pass, an electric fieldperpendicular to the plane of the gyricon with a field strength E₂ suchthat φ₁ /T>E₂ >φ₂ /T (in other words, a voltage difference V acrosssheet 1420 such that φ₁ >V>φ₂). The first pass orients all three balls1421, 1422, and 1423 with their center segments perpendicular to theplane of sheet 1420. The second pass reorients balls 1422 and 1423 sothat their center segments become parallel to the plane of gyricon sheet1420. The second pass has no effect on the orientation of ball 1421,because the applied field is less than the rotation threshold for ball1421.

In the third view of FIG. 14F, balls 1421 and 1422 are oriented withtheir center segments perpendicular to the plane of gyricon sheet 1420,and ball 1423 is oriented with its center segment parallel to the planeof gyricon sheet 1420. An observer at I sees a lightly saturated color.This orientation of the balls is obtainable by applying, in a firstpass, an electric field in the plane of gyricon with a field strength E₁>φ₁ /T (in other words, a voltage difference V such that V/W>φ₁ /T), andthereafter applying, in a second pass, an electric field perpendicularto the plane of the gyricon with a field strength E₂ such that φ₂ /T>E₂>φ₃ /T (in other words, a voltage difference V across sheet 1420 suchthat φ₂ >V>φ₃). The first pass orients all three balls 1421,1422, and1423 with their center segments perpendicular to the plane of sheet1420. The second pass reorients ball 1423 so that its center segmentbecomes parallel to the plane of gyricon sheet 1420. The second pass hasno effect on the orientation of balls 1421 and 1422, because the appliedfield is less than the rotation threshold for these balls.

In the fourth and final view of FIG. 14F, balls 1421, 1422, and 1423 allare oriented with their center segments perpendicular to the plane ofgyricon sheet 1420. An observer at I sees a minimally saturated color.This orientation of the balls is obtainable by applying an electricfield in the plane of gyricon with a field strength E >φ₁ /T, or inother words, a voltage difference V such that V/W>φ₁ /T.

From these examples, it can be seen that in order to provide variablecolor saturation with gyricon sheet 1420, a series of one or moreelectric fields can be applied. Each applied field of the series has itselectric field vector oriented in one of two directions: either in theplane of sheet 1420, or else perpendicular to the plane of sheet 1420.Color saturation is controlled by controlling the proportion of ballsoriented such that their colored center segments are parallel to theplane of sheet 1420 and thus observable to the observer at I. Each ballis in one of two dispositions: either "fully on," that is, oriented soas to make its maximal possible contribution to the observable color, orelse "fully off," that is, oriented so as to make its minimalcontribution to the observable color. In contrast with the canted-fieldtechnique described earlier, intermediate orientations are not used.

In general, a gyricon image element in which there are N different setsof gyricon balls, each set having a distinct threshold φ_(n), with eachball capable of being in one of two orientations, can provide up to2^(N) different combinations of ball orientations if N addressing passesare used (that is, if each set is addressed individually). For example,if a red subpixel of the RGB gyricon previously described with referenceto FIGS. 10A-10C has in it five sets of red-center-segment balls, eachset having a distinct rotation threshold, and each ball in the subpixelcan be oriented with its center segment either parallel to theobservable surface ("fully on") or perpendicular to the observablesurface ("fully off"), then up to (2)⁵ =32 different levels of red colorsaturation can be provided for the subpixel. Unfortunately, it is notalways practical to provide access to all 2^(N) available combinationsof ball orientations. Thus, in this example, accessing of all 32saturation levels of the red subpixel requires that each of the fivesets of balls be separately addressed, which in turn requires five-passaddressing. In general, to access any arbitrary one of the 2^(N)available combinations of ball orientations, N-pass addressing isrequired, which can be prohibitively timeconsuming for even modestvalues of N.

An alternative approach to controlling variable color saturation in amultithreshold gyricon image element provides N+1 levels of availablesaturation and requires at most two addressing passes per population ofballs. This approach works as follows: A cutoff value is selected,typically a value between two adjacent thresholds φ_(i) and φ_(i) +1.The cutoff value serves to divide the N sets of balls of the populationinto two larger groups. All balls having rotation thresholds greaterthan the cutoff value are in a first group, and all balls havingthresholds less than or equal to the cutoff value are in a second group.The two groups can be addressed in two passes: a first pass in which allballs in both the first and second groups are reset to a defaultorientation (for example, the "fully off" orientation), followed by asecond pass in which balls in the second group only are oriented in anon-default orientation (for example, the "fully on" orientation) byapplication of an electric field having a strength equal to the chosencutoff value.

An example of this alternative approach is seen in the foregoingdescription of the second and third views of FIG. 14F, in which it wasexplained how two addressing passes can be used to obtain the ballorientations shown. Expressed as a voltage to be applied across theelastomer thickness T, the cutoff value φ_(c) for the second view ofFIG. 14F is chosen such that φ₁ >φ_(c) >φ₂, and for the third view ofFIG. 14F, such that φ₂ >φ_(c) >φ₃.

Further, it will be appreciated that if the cutoff value φ_(c) is chosensuch that φ_(c) >φ₁ (for example, if φ_(c) =∞), the alternative approachcan be used to obtain the ball orientations shown in the first view ofFIG. 14F. Similarly, if the cutoff value is chosen such that φ₃ >φ_(c)(for example, if φ_(c) =0), the approach can be used to obtain the ballorientations shown in the fourth and final view of FIG. 14F. In each ofthese cases, two-pass addressing is somewhat redundant, inasmuch assingle-pass addressing would suffice. That is, for the first view ofFIG. 14F, the results of the first addressing pass are completely undoneby the second pass, and for the fourth view of FIG. 14F, the results ofthe first pass require no further correction by the second pass.Accordingly, in such cases it can be worthwhile to omit redundantaddressing steps, in order to reduce addressing time.

The alternative, cutoff-value approach to multithreshold, multipassaddressing is often to be preferred over the more general but moretime-consuming N-pass approach described previously for color presencecontrol applications. In particular, the two-pass approach worksespecially well for controlling color presence when N is large. Thenumber of available gradations of control is N+1, and the number ofaddressing passes is never more than two. Thus fine control over colorsaturation, gray scale, and the like are facilitated.

Moreover, the cutoff-value addressing approach can obviate the need forvery sharp thresholds. A nonzero value of Δ defines a range ofthresholds for a given population of balls; choosing a cutoff valueφ_(c) in this range divides the population in two. For example,referring once again to FIG. 14D, each ball in the first population hasa rotation threshold somewhere between φ₁ and φ₁ +Δ₁. A color saturation(for example) associated with the third population of balls can becontrolled by resetting all the balls of the third population to adefault orientation with an applied voltage exceeding φ₁ +Δ₁ in a firstpass, and thereafter orienting a subset of the balls to a new,non-default orientation with an applied voltage at a cutoff value φ_(c)such that φ₁ <φ_(c) <φ₁ +Δ₁ in a second pass. This can be repeated forthe balls in the second and third populations, reducing the appliedvoltage appropriately each time, until the desired saturations areestablished for each color. From this example, it can be appreciatedthat the threshold width Δ for each population can affect the degree ofprecision with which color saturation can be controlled. Assuming thatthe precision with which φ_(c) can be chosen is limited, then as Δ isreduced towards zero, there will be effectively fewer available cutoffvalues within each population and thus fewer gradations of colorsaturation control for the color associated with that population. Thusthe cutoff-value multithreshold addressing approach turns a widethreshold width Δ to best advantage; sharp thresholds are not especiallydesirable in this approach.

For the three populations of balls in FIG. 14D, at most six addressingpasses are required for the cutoff-value multithreshold addressingapproach. In general, for K populations of balls, at most 2K addressingpasses are required for this approach.

The series of views of FIG. 14G illustrates an example of thecutoff-value addressing approach as applied to a three-layer gyriconhaving three populations of three-segment balls, one population perlayer. For example, the gyricon can be a CMY gyricon. Each layer's ballpopulation has a different associated minimum threshold φ and a nonzerothreshold width Δ. In particular, it is assumed for purposes of thisexample that each layer's population of balls consists of severalsubpopulations, each subpopulation having a distinct (sharp) thresholdin the range from φ to φ+Δ.

The views of the series are all side views of a single-pixel region ingyricon sheet 1450 having thickness T (that is, each layer in sheet 1450has thickness T/3) and width W.

For a thickness T of elastomer, balls in layer 1453 have a lowestminimum threshold potential φ₃ and a nonzero threshold width Δ₃ ; ballsin layer 1452 have an intermediate threshold potential φ₂ and a nonzerothreshold width Δ₂ ; and balls in layer 1451 have a highest thresholdpotential φ₁ and a nonzero threshold width Δ₁. Each layer is to beaddressed with an electric field that can be oriented either parallel orperpendicular to the plane of sheet 1450.

The population of balls in layer 1453 includes balls 1453a, 1453b,1453c, 1453d, and 1453e, which have individual rotation thresholdsφ_(3a), φ_(3b), φ_(3c), φ_(3d), and φ_(3e), respectively, such that (φ₃+Δ₃)>φ_(3a) >φ_(3b) >φ_(3c) >φ_(3d) >φ_(3e) >φ₃. The population of ballsin layer 1452 includes balls 1452a, 1452b, 1452c, 1452d, and 1452e,which have individual rotation thresholds φ_(2a), φ_(2b), φ_(2c),φ_(2d), and φ_(2e), respectively, such that (φ₂ +Δ₂)>φ_(2a) >φ_(2b)>φ_(2c) >φ_(2d) >φ_(2e) >φ₂. The population of balls in layer 1451includes balls 1451a, 1451b, 1451c, 1451d, and 1451e; which haveindividual rotation thresholds φ_(1a), φ_(1b), φ_(1c), φ_(1d), andφ_(1e), respectively, such that (φ₁ +Δ₁)>φ_(1a) >φ_(1b) >φ_(1c) >φ_(1d)>φ_(1e) >φ₁.

In the first view of FIG. 14G, corresponding to the first addressingpass, an electric field E1₁₁ is applied in the plane of sheet 1450. Thefield is of sufficient strength to rotate all the balls in all threelayers; that is, the applied voltage V₁⊥ is such that (V₁⊥ /W)>(φ₁+Δ₁)/T. All of balls 1451a, 1451b, 1451c, 1451d, 1451e, 1452a, 1452b,1452c, 1452d, 1452e,1453a, 1453b, 1453c, 1453d, and 1453e are rotated sothat their dipole moments align with the applied field, which causestheir center segments to be oriented perpendicularly to the plane ofsheet 1450. In other words, all the balls are reset to their "fully off"orientations.

In the second view of FIG. 14G, corresponding to the second addressingpass, an electric field E1.sub.⊥ is applied perpendicular to the planeof sheet 1450. The field is of sufficient strength to rotate some of theballs in layer 1451 and all of the balls in layers 1452 and 1453; thatis, the applied voltage V1.sub.⊥ across the thickness T of sheet 1450 issuch that (φ₁ +Δ₁)>V1.sub.⊥ >φ₁. More particularly in this example, theapplied voltage V1.sub.⊥ is chosen such that balls 1451c, 1451d, and1451e are affected by the applied voltage while balls 1451a and 1451bare not. Thus φ_(1b) >V1.sub.⊥ >φ_(1c). (Put another way, V1.sub.⊥defines the cutoff value φ_(c) for the first population of balls.) Inresponse to the applied field E1.sub.⊥, balls 1451c, 1451d, and 1451e,along with all of balls 1452a, 1452b, 1452c, 1452d, 1452e,1453a, 1453b,1453c, 1453d, and 1453e, are rotated so that their dipole moments alignwith the applied field, which causes their center segments to beoriented parallel to the plane of sheet 1450. That is, all these balls1451c, 1451d, 1451e, 1452a, 1452b, 1452c, 1452d, 1452e, 1453a, 1453b,1453c, 1453d, and 1453e are oriented in their "fully on" orientations atthe end of the second pass. Balls 1451a and 1451b remain in their reset,"fully off " orientations.

In the third view of FIG. 14G, corresponding to the third addressingpass, an electric field E2₁₁ is applied in the plane of sheet 1450. Thefield is of sufficient strength to rotate all of the balls in layers1452 and 1453 while leaving all of the balls in layer 1451 unaffected;that is, the applied voltage V2₁₁ is such that (φ₁ /T)>(V2₁₁ /W)>(φ₂+Δ₂)/T. Balls 1452a, 1452b, 1452c, 1452d, 1452e, 1453a, 1453b, 1453c,1453d, and 1453e are rotated so that their dipole moments align with theapplied field, 5 which causes their center segments to be orientedperpendicularly to the plane of sheet 1450. In other words, all theballs in layers 1452 and 1453 are again reset to their "fully off"orientations, while balls in layer 1451 remain as they were. In thefourth view of FIG. 14G, corresponding to the fourth addressing pass, anelectric field E2.sub.⊥ is applied perpendicular to the plane of sheet1450. The field is of sufficient strength to rotate some of the balls inlayer 1452 and all of the balls in layer 1453, without affecting anyballs in layer 1451; that is, the applied voltage V₁⊥ across thethickness T of sheet 1450 is such that (φ₂ +Δ₂)>V2.sub.⊥ >φ₂. Moreparticularly in this example, the applied voltage V₁⊥ is chosen suchthat balls 1452b, 1452c, 1452d, and 1452e are affected by the appliedvoltage while ball 1452a is not. Thus φ_(2a) >V₂⊥ >φ_(2b). (Put anotherway, V₂⊥ defines the cutoff value φ_(c) for the second population ofballs.) In response to the applied field E2.sub.⊥, balls 1452b, 1452c,1452d, and 1452e, along with all of balls 1453a, 1453b, 1453c, 1453d,and 1453e, are rotated so that their dipole moments align with theapplied field, which causes their center segments to be orientedparallel to the plane of sheet 1450. That is, all these balls 1452b,1452c, 1452d, 1452e, 1453a, 1453b, 1453c, 1453d, and 1453e are orientedin their "fully on" orientations at the end of the fourth pass. Ball1452a remains in its reset, "fully off" orientation.

In the fifth view of FIG. 14G, corresponding to the fifth addressingpass, an electric field E3.sub.∥ is applied in the plane of sheet 1450.The field is of sufficient strength to rotate all of the balls in layer1453 while leaving all of the balls in layer 1451 and 1452 unaffected;that is, the applied voltage V3.sub.∥ is such that (φ₂ /T)>(V3.sub.∥/W)>(φ₃ +Δ₃)/T. Balls 1453a, 1453b, 1453c, 1453d, and 1453e are rotatedso that their dipole moments align with the applied field, which causestheir center segments to be oriented perpendicularly to the plane ofsheet 1450. In other words, all the balls in layer 1453 are yet againreset to their "fully off" orientations, while balls in layers 1451 and1452 remain as they were.

In the sixth and final view of FIG. 14G, corresponding to the sixthaddressing pass, an electric field E3.sub.⊥ is applied perpendicular tothe plane of sheet 1450. The field is of sufficient strength to rotatesome of the balls in layer 1453, without affecting any balls in layers1451 and 1452; that is, the applied voltage V₃⊥ across the thickness Tof sheet 1450 is such that (φ₃ +Δ₃)>V₃⊥ >φ₃. More particularly in thisexample, the applied voltage V₃⊥ is chosen such that ball 1453e isaffected by the applied voltage while balls 1453a, 1453b, 1453c, and1453d are not. Thus φ_(3d) >V₃⊥ >φ_(3e). (Put another way, V₃⊥ definesthe cutoff value φ_(c) for the third population of balls.) In responseto the applied field E3.sub.⊥, ball 1453e is rotated so that its dipolemoment aligns with the applied field, which causes its center segment tobe oriented parallel to the plane of sheet 1450. None of the other ballsis affected.

This completes the addressing sequence of FIG. 14G. After the sixth passis complete, an observer at I sees a pixel in which the color providedby the center segments of the balls in layer 1451 is moderatelysaturated, the color provided by the center segments of the balls inlayer 1452 is heavily saturated, and the color provided by the centersegments of the balls in layer 1453 is very lightly saturated. Again, itis worth noting that although the balls are illustrated in FIG. 14G ashaving five discrete thresholds and neatly arranged in order ofdecreasing rotation threshold, this is done only for purposes ofclarifying the exposition. In practice, each population of balls willhave a large number of thresholds, which will be statisticallydistributed over the interval between φ₁ and (φ₁ +Δ₁) for layer 1451,over the interval between φ₂ and (φ₂ +Δ₂) for layer 1452, and over theinterval between φ₃ and (φ₃ +Δ₃); and balls of these differentthresholds will be spatially distributed throughout their respectivelayers.

The parallel and perpendicular addressing fields used in FIGS. 14F and14G can be generated separately for each pixel or other image element,using an electrode configuration that is similar in appearance to thecanted-field electrode configuration previously described with referenceto FIG. 8A. However, only parallel and perpendicular fields are needed,so the voltages V1, V2, V3, and V4 can be constrained such that eitherV1=V2 and V3=V4, or else V1=V3 and V2=V4. Thus the voltage controlcircuitry can be simplified as compared with the control circuitrynecessary to provide a fully general canted-field capability.

Alternatively, the parallel and perpendicular fields can be generatedwith the less complex and less expensive electrode configurationdepicted in FIG. 8F, in which the in-plane "erase" field is applied tothe entire gyricon sheet at once, and only the perpendicular field isseparately addressable for each image element. This configuration workswell with the cutoff-value approach to multithreshold, multipassaddressing as exemplified in FIG. 14G, because if the defaultorientation is "fully off" then the first addressing pass for eachpopulation of balls in every pixel is always a bulk erasure. The secondpass, which turns some of the balls to "fully on," can vary in appliedvoltage from pixel to pixel. The electrode configuration of FIG. 8F isnot sufficient for more general N-pass approach in which all 2^(N)possible combinations of ball orientations are to be made accessible.

It should be noted in conjunction with the multithreshold approaches forcolor presence control that if the different rotation thresholds for theballs of each color in a gyricon are achieved by using balls ofdifferent sizes, the choice of which balls should be larger and whichballs should be smaller can depend on the number of steps of presenceresolution required for each color. For example, suppose that in amultilayer CMYK gyricon, balls in the cyan layer have a first meanradius, balls in the magenta layer have a second mean radius, balls inthe yellow layer have a third mean radius, and balls in the black layerhave a fourth mean radius. It is advantageous in this case for the ballshaving the largest radius to be in the yellow layer, and the ballshaving the smallest radius to be in the black layer, because typicallythe human eye resolves more gradations of gray-scale than gradations ofcolor saturation, and resolves gradations of yellow less well thangradations of other colors. If multithresholding is used, the number ofavailable gradations for a given color in a given pixel depends on thenumber of separately addressable balls of that color in the pixel; themore balls of a given color, the finer the control that can be had overthe presence of that color in the final color mix. Thus, since the leastprecise control is required for yellow and the most precise control isrequired fro black, there can be relatively fewer yellow balls per pixelas compared with the number of cyan or magenta balls per pixel, andrelatively more black balls per pixel as compared with the number ofcyan or magenta balls per pixel (that is, N_(yellow) <N_(cyan)<N_(black) and N_(yellow) <N_(magenta) <N_(black)).

The multithreshold, multipass techniques illustrated in FIGS. 14F-14Gcan be usefully compared with the canted-field techniques describedearlier with reference to FIGS. 8A-8C. The two sets of techniquesprovide two distinct sets of approaches to controlling the degree ofpresence (e.g., color saturation, gray-scale level, etc.) of any givencolor in any single image element of a gyricon. Briefly summarized,these two sets of approaches can be contrasted as follows:

The canted-field approaches work by varying the angle of each ball withrespect to the gyricon's observable surface, and thus the degree towhich each ball contributes to the observable color. Each ball can berotated by the canted field to any angle of a continuous range ofangles. All balls in a given region are rotated at once. Addressingtakes place in a single operation.

The multithreshold, multipass approaches work by varying the proportionof balls rotated, and thus the number of balls available to contributeto the observable color. Each ball can be rotated to one of twopositions, either "fully on" (maximum contribution to the observablecolor) or "fully off" (minimum contribution to the observable color);unlike the canted-field approach, there are no intermediate positions.Not all balls in a given region need be rotated at once. Addressingtakes place in series of passes; for example, all balls can be reset tothe "fully off" orientation in a first pass, and then a subset of ballscan be oriented in the "fully on" orientation in a second pass.

As previously mentioned with reference to FIG. 14E, multithreshold andcanted field techniques can be used together in a single gyricon, withmultithresholding being used to select particular groups (e.g., layers)of balls and canted fields being used to control color presence withineach selected group.

Fabrication Techniques for Strategic Placement of Different Balls in aGyricon Sheet

The RGB gyricon of FIGS. 10A-10C is constructed from three differentkinds of balls, namely, balls with red center segments, balls with greencenter segments, and balls with blue center segments. These threedifferent kinds of balls are placed in different subpixel regions in thegyricon sheet. A red subpixel contains balls with red center segmentsonly, and does not contain balls of the other two kinds. Similarly, agreen subpixel contains balls with green center segments only, and ablue subpixel contains balls with blue center segments only. To buildthis gyricon, then, requires a manufacturing technique for placing thedifferent kinds of balls in their respective different locations in theelastomer sheet, so that the desired geometric pattern of red, green,and blue subpixels (e.g., the pattern of FIG. 10C) is obtained.

There are other occasions when it is desirable to create a display fromassembled patches of distinctly colored gyricon balls. As an example, inthe case of an automobile display, the speedometer might be displayedusing bichromal red and white balls; the odometer, a region of green andwhite bichromal balls; the fuel gauge, black and white bichromal balls;and the tachometer fluorescent blue and white bichromal balls. Yetanother example would be in a decoratively patterned gyricon-basedarchitectural screen, made according to the principles previouslydescribed with reference to FIGS. 7A and 7E. For example, a pattern ofdifferent balls having different kinds of transparent center segments(e.g., some clear, others "smoke-glass" colored, still others tintedpink or another chromatic color), might be desired.

In general, there can be various circumstances in which it is necessaryor advantageous to place different kinds of gyricon balls at differentpreferred chosen locations in the elastomer layer during themanufacturing process. By "different kinds" is meant any physicaldistinctions between balls of one set and balls of another set,including different optical properties (of which color is only oneexample) and distributions of optical properties among regions withinthe balls; any and all of the aforementioned electrical, mechanical,structural, and material properties, such as size, shape, electricalmonopole and dipole moments, and so forth, that were previouslymentioned as being among the properties that can affect ball rotationthresholds; and, in general, any other physical characteristics that canbe used to differentiate between different balls, such as, for example,ferromagnetic properties in gyricon balls that have such properties (seeU.S. Pat. No. 4,126,854, incorporated hereinabove by reference, at col.6, lines 16-30, for an example of this).

Various techniques can be used to obtain patterned or other strategicball placement during manufacture of the gyricon elastomer sheet. Onesuch technique is a nonfusing xerographic technique in which the desiredpattern of gyricon balls of different kinds is xerographically "printed"on a partially cured elastomer using "toners" that comprises the gyriconballs themselves. In this manner, different kinds of gyricon balls canbe placed at any desired locations on the partially cured sheet. Oncethe balls are placed as desired, additional elastomer material inuncured liquid form is poured over them so that the resulting elastomersheet has the gyricon balls disposed inside it rather than on top of it.

The xerographic technique is informed by the observation that thespheroidal gyricon balls are, in certain ways, very much like the tonerparticles used in conventional xerography. In particular, they aredielectric and easily triboelectrically charged, like toner particles,and typically they are about the same size as toner particles. Thismeans that the gyricon balls can be placed in a xerographic developmentsystem, in place of ordinary toner, and if the development system issubsequently placed in a xerographic engine the latter can produceimages made from the balls.

A common form of xerographic development system works by mixing tonerparticles with steel or ferrite (magnetic) beads in a sump. In theprocess of mixing the toner particles with the steel or ferrite beads,the toner particles develop a triboelectric charge. A fraction of thismixture of toner particles and beads is brushed against the surface of aphotoconductor drum that has an imagewise distribution of charges of theopposite polarity on its surface. This can be obtained by uniformlycharging the surface of the photoconductor drum with ions from a coronadischarge apparatus and subsequently imagewise discharging thephotoconductor by exposing it to light from an image, as is wellunderstood in the xerographic arts. The toner particles adhere to areasof the photoconductor drum that have a high density (voltage) of chargeof the opposite polarity. This creates an imagewise toner image.

In conventional xerography, the toner image formed on the photoconductordrum is subsequently transferred to paper, usually by placing a sheet ofpaper in contact with the photoconductor drum and placing another coronadischarge apparatus on the opposite side of the paper, attracting thetoner to the paper surface. Thereafter, the toner is fused (melted) intothe paper. Here, of course, it is preferred not to melt the gyriconballs, and the preferred receiving surface is not paper but rather theelastomer sheet of the gyricon itself. Accordingly, a nonfusingxerographic process is used. (Other nonfusing xerographic processes areknown; see, for example, U.S. Pat. No. 5,075,186, incorporatedhereinabove by reference). Toner made from gyricon balls is imaged ontoa photoconductor drum and is transferred from the photoconductor onto anadhesive receiving medium, which can conveniently be made of elastomermaterial in a sticky, partially cured state.

An example of a nonfusing xerographic color printer 1500 suitable forgyricon ball placement is shown in FIG. 15A. For purposes of discussionof FIG. 15A it will be assumed that three sets of gyricon balls, onered, one green, and one blue (e.g., three-segment balls with red, green,and blue center segments, respectively) are to be placed in the gyriconsheet, it being understood that any two or more sets can be placed withthis technique.

A photoconductor drum 1505 is exposed to a first laser light image,which imagewise discharges drum 1505. Laser light for the image isproduced by scanning laser 1502 in conjunction with mirror 1503 and lens1504, in a manner like that used in known laser printing and digitalxerographic techniques. As drum 1505 rotates counterclockwise (in thedirection of arrow a), red development housing 1510, which contains amixture of ferrite beads and toner made from the red balls, is moved (asindicated by arrows d) into near contact with drum 1505. The mixture ofmagnetic beads and toner brushes the surface of the photoconductor drum1505. A magnetic field (not shown) holds onto the magnetic beads. A biasvoltage between development housing 1510 and drum 1505 allows the toner(here, the red balls) to stick to the surface of drum 1505 only in thoseareas of the photoconductor drum where the charge has previously beenremoved by exposure to the first laser light image. In this manner animagewise layer of red balls is built up on the surface of thephotoconductor drum. This image 1526 is next transferred to a storagedrum 1525 by creating a high electrical field between the surface ofphotoconductor drum 1505 and the surface of storage drum 1525. Storagedrum 1525 rotates (as indicated by arrow b) in the opposite direction todrum 1505. The image 1526, formed of the red balls, is shown stored onstorage drum 1525.

Next, the photoconductor drum 1505 is again uniformly charged by meansof a corona discharge apparatus and this time it is discharged by asecond laser light image, again produced with laser 1502. This time,green development housing 1511, which contains a mixture of ferritebeads and toner made from the green balls, is engaged, and it imagewisedeposits green balls on the surface of photoconductor drum 1505 in thesame manner as was done previously for the red ball image 1526. Thegreen ball image 1527, here seen while still present on drum 1505, istransferred to storage drum 1525 in such a way that it exactlysuperimposes on the red ball image 1526 that is already there.

In like manner, a third image (not shown) made from toner from bluedevelopment housing 1512 can be produced on photoconductor drum 1505 andtransferred to storage drum 1525, exactly superimposed on the previouslysuperimposed red and green ball images 1526,1527.

When all three (or more) colored images have been accumulated on thesurface of storage drum 1525, the images are transferred to a receivingsurface 1530. In a conventional xerographic printer, the receivingsurface is normally paper, and the next step thereafter is heatfusing ofthe toner image to paper. Here, the receiving surface is an adhesivesurface that will position the balls for inclusion in the elastomerlayer of the gyricon, and there is no fusing step.

It has been found that a thin layer of partially cured SYLGARD 184elastomer, a preferred elastomer material for making gyricon sheets, isvery sticky. If receiving surface 1530 is a surface of partially curedelastomer, and this surface is moved (arrows c) in the same direction asthe surface of storage drum 1525, at the same surface speed, and isallowed to come very close to the surface of storage drum 1525, asignificant fraction of the colored ball image stored on storage drum1525 will transfer to receiving surface 1530. (The surface of storagedrum 1525 can advantageously be coated with a non-stick substance, suchas TEFLON, so that it can actually be placed in direct contact with thesticky elastomer of receiving surface 1530. ) If a strong electricalfield is placed across these two surfaces, an even larger fraction ofthe colored ball image will transfer.

Pouring uncured elastomer onto the surface of the transferred coloredball image, removing the trapped air (for example, by the application ofa vacuum or the use of a centrifuge), and curing the elastomer willresult in encapsulation of the colored ball image. Thus thesuperposition of colored ball images that has been formed on storagedrum 1525 becomes the pattern of balls in the elastomer sheet of thegyricon. After plasticization by application of a dielectric plasticizerfluid to swell the elastomer sheet, rendering the balls free to rotatetherein, the gyricon will be ready for use.

FIG. 15B is a highly magnified view of a powder mixture of toner andbeads for use in the development housings 1510, 1511, 1512 of thexerographic apparatus of FIG. 15A. Powder 1515 includes a large numberof gyricon balls 1516 mixed together with a large number of beads 1517made from ferrite or other magnetic substance. Beads 1517 serve toimpart triboelectric charge to balls 1516, in a manner similar to thatin which in which ferrite beads serve to impart triboelectric charge toparticles of a dry ink or other marking substance in conventionalxerographic toner. Typically, the number of beads 1517 will beapproximately equal to the number of balls 1516, and the beads 1517 arealso spheroidal but about an order of magnitude larger in size thanballs 1516. However, it will be understood that different kinds ofgyricon balls, different bead materials and sizes, and differentproportions of balls to beads in the mixture can be used, as appropriatefor the particular application.

FIG. 15C illustrates the step of pouring the uncured elastomer onto thetransferred colored ball image. A section 1542 of partially curedelastomer from receiving surface 1530, onto which the colored ball imagehas been transferred from storage drum 1525, has been removed to aholding platform 1538 and placed between retaining walls 1539a, 1539b asshown. Balls 1545 are the gyricon balls that make up the transferredcolored ball image. Uncured elastomer 1541, which is a liquid, isdispensed from vessel 1540 onto the partially cured elastomer section1542 and over balls 1545, in such a manner asto cover balls 1545 whilenot moving them from their respective positions in the elastomer. Thusthe colored ball image formed of balls 1545 remains undisturbed as theadditional uncured elastomer 1541 is poured over it. Retaining walls1539a, 1539b hold the dispensed uncured elastomer in place during thecuring process.

The xerographic ball placement technique is useful for fabricating anygyricon that includes two or more distinct kinds of balls that are notuniformly distributed throughout the entirety of the elastomer material.Another technique for obtaining low cost, imagewise colored balldistributions takes advantage of the fact that the gyricon balls arehighly spherical and, in the absence of electrostatic charges on theirsurfaces, exhibit excellent flow characteristics. Thus a kind of "silkscreening" is possible.

The silk screen ball placement technique is illustrated in FIG. 15D.Balls 1575 are dispensed from dispenser 1570 onto a screen 1580 that isdisposed above a sticky layer 1590 of partially cured elastomer. Screen1580 has holes that define the desired image or pattern in which balls1575 are to be placed in the gyricon sheet. The holes are large enoughfor balls to 1575 to pass through, yet small enough to give the desiredresolution of ball placement. Balls 1575 are placed on screen 1580 and,with appropriate vibration supplied by agitator 1581, pass through theholes of screen 1580 in an imagewise manner. Upon impacting the surfaceof the partially cured elastomer layer 1590, balls 1575 are stuck to thesurface.

The foregoing process can be repeated, using different screens fordifferent kinds of balls, until the desired pattern of different ballsis placed on the elastomer surface. For example, a first silk screen canbe used to place red balls in elastomer layer 1590, and thereafter asecond silk screen can be used to place green balls in elastomer layer1590. An additional screening step is used for each additional color.Finally, when all the balls are in place, uncured elastomer can bepoured over the surface, in a manner similar to that which was shown inFIG. 15C, so as to cover over the placed balls. Next, trapped air isremoved from the elastomer, which is then ready to be cured andplasticized.

Conclusion

The foregoing specific embodiments represent just some of thepossibilities for practicing the present invention. Many otherembodiments are possible within the spirit of the invention. Forexample:

A gyricon used in a full-color display or full-color electric paperapplication need not be restricted to conventional RGB or CMY/CMYK colorschemes. To improve the color gamut, additional colors can beincorporated. Moreover, as indicated above with regard to the highlightcolor application, a special custom color can be provided, for exampleto ensure accurate rendering of a company logo.

The electrical anisotropy of a gyricon ball need not be based on zetapotential. It is sufficient that there is an electrical dipole momentassociated with the ball, the dipole moment being aligned with respectto the ball in such a way as to facilitate a useful rotation of the ballin the presence of an applied external electric field. (Typically, thedipole moment is oriented along an axis of symmetry of the ball.)Further, it should be noted that a gyricon ball can have an electricalmonopole moment in addition to its electrical dipole moment, as forexample when the dipole moment arises from a separation of two positivecharges of different magnitudes, the resulting charge distribution beingequivalent to a positive electrical monopole superposed with aelectrical dipole.

Although the gyricon balls that have been described above arerotationally responsive to DC addressing voltages (whereas thosedisclosed by Goodrich in U.S. Pat. No. 4,261,653 are not), these ballscan also respond to certain AC addressing voltages. In particular,multisegmented zeta-potential-based gyricon balls are suitable for usein raster-scanned addressable displays operating at video frame rates.Moreover, it will be appreciated that certain aspects of the presentinvention are adaptable even to gyricons in which the balls arerotationally responsive only to non-DC voltages (e.g., RF voltages inGoodrich's case).

The optical anisotropy of a gyricon ball need not be based on color.Other optical properties can vary as different aspects of the gyriconball are presented to an observer, including (but not limited to)polarization, birefringence, phase retardation, light scattering, andlight reflection. In general, the gyricon balls can be used to modulatelight in a wide variety of ways.

The incident light that encounters a gyricon need not be restricted tovisible light. Given suitable materials for the gyricon balls, theincident "light" can be, for example, infrared light or ultravioletlight, and such light can be modulated by the gyricon.

On several occasions the foregoing description refers to a planargyricon sheet and to electric fields that are parallel to the sheet, inthe plane of the sheet, perpendicular to the sheet, at a specified angleto the sheet, and so forth. However, persons of skill in the art willappreciate that a gyricon sheet made of a flexible material can betemporarily or permanently deformed (for example, flexed, folded, orrolled) so as not to be strictly planar overall. In such cases, electricfield angles can be measured, for example, with respect to the sheet ina locally planar neighborhood that includes the gyricon ball or balls ofinterest. Also, it will further be apprecated that in practice theelectric fields can vary somewhat from the parallel, perpendicular, andother angles described, for example, due to manufacturing tolerances orslight imperfections of particular gyricon sheets and electrodeassemblies.

The gyricon's paper-like advantages of flexibility, light weight, and soforth make it particularly useful for electric paper applications.However, as noted earlier, the gyricon can also be used in rigid orfixed flat-panel displays, such as for computer screens, automobiledashboards, display signs, etc. Moreover, as seen above with regard toelectric Venetian blinds and windowshades, a gyricon need not be used asan information display medium. The light-modulating capabilitiesprovided by the gyricon of the present invention can find many otherapplications.

The canted-field and multithreshold techniques described hereinabovelend themselves to further applications. One possibility is to usecanted-field electrodes in conjunction with an elastomer sheetcontaining black-and-white gyricon balls of the prior art. The cantedfields can rotate the balls to any desired angle, that is, any desiredmixture of black and white, thereby making the gyricon capable ofgray-scale imaging. Another possibility is to write on RGBmultithreshold electric paper with a voltage source, such as a poweredstylus, that provides three distinct voltages or voltage ranges. Thisallows the user to write on electric paper in three different colors.

Full-color gyricons have been described hereinabove that provide colorsaturation control, for example by way of canted fields andmultithresholding techniques. However, a full-color pixel-addressablegyricon that provides only two saturations of each color per pixel,namely, fully saturated or minimally saturated, and does not providevariable color saturation control, can nevertheless be useful. Inparticular, a CMY display can be built that is suitable for halftonecolor applications.

Accordingly, the scope of the invention is not limited to the foregoingspecification, but instead is given by the appended claims together withtheir full range of equivalents.

The claimed invention is:
 1. A spheroidal ball having a center point andcomprising seven (7) segments arrayed substantially parallel to oneanother, each segment being adjacent to at least one other segment andto no more than two other segments, each segment adjacent to exactly oneother segment being an exterior segment, each segment adjacent toexactly two other segments being an interior segment, adjacent segmentsbeing adjoined to one another at substantially planar interfaces, theseven segments including:a central segment including the center point ofthe ball, the central segment being an interior segment and having athickness, the central segment being transparent; a first exteriorsegment, the first exterior segment being transparent; a second exteriorsegment situated opposite the first exterior segment with respect to thecentral segment, the second exterior segment being transparent; a firstinterior segment adjacent to the first exterior segment, the firstinterior segment having a thickness less than the central segmentthickness, the first interior segment having a first color; a secondinterior segment adjacent to the first interior segment and to thecentral segment, the second interior segment having a thickness lessthan the central segment thickness, the second interior segment having asecond color; a third interior segment situated opposite the secondinterior segment with respect to the central segment, the third interiorsegment having a thickness less than the central segment thickness, thethird interior segment having a color different from the first color;and a fourth interior segment adjacent to the third interior segment andsituated opposite the first interior segment with respect to the centralsegment, the fourth interior segment having a thickness less than thecentral segment thickness, the fourth interior segment having a colordifferent from the second color and different from the color of thethird interior segment;the ball having an anisotropy for providing anelectrical dipole moment, the electrical dipole moment rendering theball electrically responsive such that when the ball is rotatablydisposed in an electric field while the electrical dipole moment of theball is provided, the ball tends to rotate to an orientation in whichthe electrical dipole moment aligns with the field.
 2. The ball of claim1 wherein the third interior segment has a third color and the fourthinterior segment has a fourth color.
 3. The ball of claim 1 wherein eachof the first, second, third, and fourth interior segments has an opaquecolor.
 4. The ball of claim 1 wherein at least one of the first, second,third, and fourth interior segments has a chromatic color.
 5. A materialcomprising:a substrate; and a plurality of the balls of claim 1 disposedin the substrate.
 6. The material of claim 5 and further comprising abacking material joined to the substrate.
 7. The material of claim 5wherein the substrate comprises a layer having a front surface and arear surface, and further comprising a backing material joined to therear surface, the backing material having a background color, thebackground color being any color different from the first and secondcolors and different from the colors of the third and fourth interiorsegments.
 8. The material of claim 7 wherein the background color iswhite and wherein each of the first, second, third, and fourth segmentshas a chromatic color selected from the group consisting of red, green,black, and blue.
 9. Apparatus comprisinga piece of the material of claim5; and means for producing an electric field to facilitate a rotation ofat least one ball rotatably disposed in the substrate of the piece ofmaterial.
 10. The apparatus of claim 9 wherein the field-producing meanscomprises means for selectively addressing a set of balls rotatablydisposed in the substrate, the set comprising at least one ball, byapplying the electric field to the balls of the set and therebyfacilitating a rotation of at least one ball of the set.
 11. Apparatuscomprising:a substrate having a surface; a plurality of spheroidal ballsdisposed in the substrate,each ball having five (5) observable aspects,each aspect being associated with a color observable by an observersituated favorably to observe the substrate surface, the aspectsincludinga first aspect associated with a first color, a second aspectassociated with a combination of the first color and another colordifferent from the first color, a third aspect associated with acombination of a second color and another color different from thesecond color, a fourth aspect associated with a color different from thesecond color, and a fifth aspect associated with a background color, thebackground color being any color with which none of the first, second,third, and fourth aspects is associated, each ball having an anisotropyfor providing an electrical dipole moment, the electrical dipole momentrendering the ball electrically responsive such that when the ball isrotatably disposed in an electric field while the electrical dipolemoment of the ball is provided, the ball tends to rotate to anorientation in which the electrical dipole moment aligns with thefield;and means for producing an electric field to facilitate a rotationof a ball rotatably disposed in the substrate (said ball for which therotation is facilitated hereinafter being referred to as "therotation-facilitated ball") so as to present, to the observer situatedfavorably to observe the substrate surface, any one of the five aspectsof the rotation-facilitated ball, thereby rendering observable to theobserver at least one color with which the presented aspect isassociated.
 12. The apparatus of claim 11 wherein the field-producingmeans comprises:means for selecting a preferred color choice, thepreferred color choice being selected from the group consisting ofthefirst color, the color combination with which the second aspect isassociated, the color combination with which the third aspect isassociated, the color with which the fourth aspect is associated, andthe background color; and means for facilitating the rotation of therotation-facilitated ball so as to present to the observer a preferredaspect of the rotation-facilitated ball, the preferred aspect being anaspect of the rotation-facilitated ball associated with the selectedpreferred color choice.
 13. The apparatus of claim 11 wherein:each ballof the plurality comprises five (5) component regions includinga firstcomponent region of the ball having a first optical modulationcharacteristic such that the first component region is of the firstcolor, a second component region of the ball having a second opticalmodulation characteristic such that the second component region is ofthe second color, a third component region of the ball having an opticalmodulation characteristic such that the third component region is of thecolor other than the first color with which the second aspect isassociated, a fourth component region of the ball having an opticalmodulation characteristic such that the third component region is of thecolor other than the second color with which the third aspect isassociated, and a fifth component region of the ball having an opticalmodulation characteristic such that the fifth component region istransparent;and the field-producing means comprises means forfacilitating the rotation of the rotation-facilitated ball so as torender observable to the observer any of the five component regions ofthe rotation-facilitated ball.
 14. The apparatus of claim 11 whereineach ball of the plurality is a multisegmented ball having a centerpoint and comprising a plurality of segments arrayed substantiallyparallel to one another, each segment being adjacent to at least oneother segment and to no more than two other segments, each segmentadjacent to exactly one other segment being an exterior segment, eachsegment adjacent to exactly two other segments being an interiorsegment, adjacent segments being adjoined to one another atsubstantially planar interfaces, the plurality of segments including:acentral segment including the center point of the ball, the centralsegment being an interior segment and having a thickness, the centralsegment being transparent; a first transparent exterior segment; asecond transparent exterior segment situated opposite the first exteriorsegment with respect to the central segment; a first interior segmentadjacent to the first exterior segment, the first interior segmenthaving a thickness less than the central segment thickness, the firstinterior segment having a first color; a second interior segmentadjacent to the first interior segment and to the central segment, thesecond interior segment having a thickness less than the central segmentthickness, the second interior segment having a second color; a thirdinterior segment situated opposite the second interior segment withrespect to the central segment, the third interior segment having athickness less than the central segment thickness, the third interiorsegment having a color different from the first color; and a fourthinterior segment adjacent to the third interior segment and situatedopposite the first interior segment with respect to the central segment,the fourth interior segment having a thickness less than the centralsegment thickness, the fourth interior segment having a color differentfrom the second color and different from the color of the third interiorsegment.
 15. The apparatus of claim 11 wherein the produced electricfield has an electric field vector, and wherein the field-producingmeans comprises:means for producing an electric field having an electricfield vector oriented at an orientation angle, the orientation anglebeing measured with respect to a vector normal to a planar portion ofthe substrate surface in a vicinity of the rotation-facilitated ball,the orientation angle being adjustable over a continuous range of anglesfrom 0 degrees to 180 degrees,the produced electric field tending torotate the rotation-facilitated ball when the electrical dipole momentof the rotation-facilitated ball is provided such that the electricaldipole moment of the rotation-facilitated ball aligns with the electricfield vector.
 16. The apparatus of claim 15 wherein the field-producingmeans further comprises:means for selecting a preferred direction oforientation for the rotation-facilitated ball by adjusting theorientation angle to a preferred angle, the preferred angle being anangle such that the electrical dipole moment of the rotation-facilitatedball aligns with the electric field vector, thereby orienting therotation-facilitated ball in the preferred direction of orientation. 17.The apparatus of claim 11 wherein the substrate comprises a layer havingfront and rear surfaces, the substrate surface being identified with thefront surface of the layer, the apparatus further comprising:a backingmaterial joined to the rear surface, the backing material being of thebackground color, the backing material being observable by the observerthrough a transparent component of the rotation-facilitated ball whenthe fifth aspect of the rotation-facilitated ball is presented to theobserver.
 18. A method comprising the steps of:providing light from alight source incident on a modulating device comprising a plurality ofelectrically anisotropic spheroidal balls disposed in a substrate havinga surface, each ball of the plurality having five (5) observableaspects, each aspect being associated with a color observable by anobserver situated favorably to observe the substrate surface, theaspects includinga first aspect associated with a fully saturated firstobservable color, a second aspect associated with a partially saturatedobservable color different from the first observable color, a thirdaspect associated with a partially saturated second observable color, afourth aspect associated with a fully saturated observable colordifferent from the second observable color, and a fifth aspect providedby a transparent component of the ball; applying an electric field in avicinity of a ball rotatably disposed in the substrate, therebyfacilitating a rotation of the ball so as to present, to the observersituated favorably to observe the substrate surface, any one of the fiveaspects of the ball for which the rotation is facilitated; andmodulating with the modulating device at least a portion of the lightincident on the modulating device, so as to provide a modulated light.19. The method of claim 18 wherein the step of applying an electricfield to facilitate a ball rotation comprises the step of:applying anelectric field having an electric field vector oriented at a preferredangle with respect to a vector normal to a planar portion of thesubstrate surface in the vicinity of the ball for which the rotation isfacilitated, the preferred angle being adjustable over a continuousrange of angles from 0 degrees to 180 degrees.
 20. The method of claim18 wherein the ball for which the rotation is facilitated comprises aplurality of segments arrayed substantially parallel to one another,each segment being adjacent to at least one other segment and to no morethan two other segments, adjacent segments being adjoined to one anotherat substantially planar interfaces having normal vectors, and whereinthe step of applying an electric field to facilitate a ball rotationcomprises:rotating the ball for which the rotation is facilitated to anorientation in which a vector normal to a planar interface of said ballis oriented at a preferred angle with respect to a vector normal to aplanar portion of the substrate surface in the vicinity of said ball,the preferred angle being adjustable over a continuous range of anglesfrom 0 degrees to 180 degrees.
 21. In an apparatus comprising aplurality of electrically anisotropic spheroidal balls rotatablydisposed in a substrate having front and rear surfaces, each ball havinga center point and comprising seven (7) segments arrayed substantiallyparallel to one another, each segment being adjacent to at least oneother segment and to no more than two other segments, each segmentadjacent to exactly one other segment being an exterior segment, eachsegment adjacent to exactly two other segments being an interiorsegment, adjacent segments being adjoined to one another atsubstantially planar interfaces, the seven segments includinga centralsegment including the center point of the ball, the central segmentbeing an interior segment and having a thickness, the central segmentbeing transparent, a first exterior segment, the first exterior segmentbeing transparent, a second exterior segment situated opposite the firstexterior segment with respect to the central segment, the secondexterior segment being transparent, a first interior segment adjacent tothe first exterior segment, the first interior segment having athickness less than the central segment thickness, the first interiorsegment having a first color, a second interior segment adjacent to thefirst interior segment and to the central segment, the second interiorsegment having a thickness less than the central segment thickness, thesecond interior segment having a second color, a third interior segmentsituated opposite the second interior segment with respect to thecentral segment, the third interior segment having a thickness less thanthe central segment thickness, the third interior segment having a colordifferent from the first color, and a fourth interior segment adjacentto the third interior segment and situated opposite the first interiorsegment with respect to the central segment, the fourth interior segmenthaving a thickness less than the central segment thickness, the fourthinterior segment having a color different from the second color anddifferent from the color of the third interior segment,the apparatushaving an array of addressable elements, each array element including atleast one ball of the plurality, a method comprising the steps of:selecting an array element; selecting a preferred direction oforientation for balls of the selected array element, the direction oforientation forming an angle with a vector normal to a planar portion ofthe substrate surface in a vicinity of the selected array element, theangle being adjustable over a continuous range of angles from 0 degreesto 180 degrees; and aligning balls of the selected array element withthe preferred direction of orientation by applying an electric field inthe vicinity of the selected array element, the electric field having anelectric field vector oriented parallel to the selected preferreddirection, thereby influencing balls of the selected array element torotate so as to align with the preferred direction of orientation, theplanar interfaces of the balls thus aligned being disposed substantiallyperpendicular to the preferred direction of orientation.
 22. The methodof claim 21 wherein:the selecting step comprises selecting a preferreddirection of orientation parallel to the planar portion of the substratesurface in the vicinity of the selected array element; and the aligningstep comprises applying an electric field to cause balls of the selectedarray element to rotate so as to present, to an observer situatedfavorably to observe the substrate surface in the vicinity of theselected array element, an aspect associated with the first color, thepreferred direction of orientation being such thata) the planarinterfaces between the segments of the balls aligned with the preferreddirection of orientation are substantially parallel to said planarportion of the substrate surface in the vicinity of the selected arrayelement, and b) the first interior segments of the balls in thepreferred orientation are nearer to the substrate surface than are thesecond, third, and fourth interior segments of said balls, so that thefirst interior segments of said balls are visible to the observer andthe second, third, and fourth interior segments are hidden from theobserver.
 23. The method of claim 21 wherein:the selecting stepcomprises selecting a preferred direction of orientation oblique to theplanar portion of the substrate surface in the vicinity of the selectedarray element; and the aligning step comprises applying an electricfield to cause balls of the selected array element to rotate so as topresent, to an observer situated favorably to observe the substratesurface in the vicinity of the selected array element, an aspectassociated with a mixture of colors, the colors of the mixture includingthe first color and the color of the third interior segment, thepreferred direction of orientation being such thata) the planarinterfaces between the segments of the balls aligned with the preferreddirection of orientation are oblique to said planar portion of thesubstrate surface in the vicinity of the selected array element, and b)the first and third interior segments of said balls are both at leastpartially visible to the observer and the second and fourth interiorsegments are hidden from the observer.
 24. The method of claim 21wherein:the selecting step comprises selecting a preferred direction oforientation oblique to the planar portion of the substrate surface inthe vicinity of the selected array element; and the aligning stepcomprises applying an electric field to cause balls of the selectedarray element to rotate so as to present, to an observer situatedfavorably to observe the substrate surface in the vicinity of theselected array element, an aspect associated with a mixture of colors,the colors of the mixture being the second color and the color of thefourth interior segment, the preferred direction of orientation beingsuch thata) the planar interfaces between the segments of the ballsaligned with the preferred direction of orientation are oblique to saidplanar portion of the substrate surface in the vicinity of the selectedarray element, and b) the second and fourth interior segments of saidballs are both at least partially visible to the observer and the firstand third interior segments are hidden from the observer.
 25. The methodof claim 21 wherein:the selecting step comprises selecting a preferreddirection of orientation parallel to the planar portion of the substratesurface in the vicinity of the selected array element; and the aligningstep comprises applying an electric field to cause balls of the selectedarray element to rotate so as to present, to an observer situatedfavorably to observe the substrate surface in the vicinity of theselected array element, an aspect associated with the color of thefourth interior segment, the preferred direction of orientation beingsuch thata) the planar interfaces between the segments of the ballsaligned with the preferred direction of orientation are substantiallyparallel to said planar portion of the substrate surface in the vicinityof the selected array element, and b) the fourth interior segments ofthe balls in the preferred orientation are nearer to the substratesurface than are the first, second, and third interior segments of saidballs, so that the fourth interior segments of said balls are visible tothe observer and the first, second, and third interior segments arehidden from the observer.
 26. The method of claim 21 wherein:theselecting step comprises selecting a preferred direction of orientationperpendicular to the planar portion of the substrate surface in thevicinity of the selected array element; and the aligning step comprisesapplying an electric field to cause balls of the selected array elementto rotate so as to present, to an observer situated favorably to observethe substrate surface in the vicinity of the selected array element, atransparent aspect, the preferred direction of orientation being suchthat the planar interfaces between the segments of the balls alignedwith the preferred direction of orientation are substantiallyperpendicular to said planar portion of the substrate surface in thevicinity of the selected array element.
 27. The method of claim 26wherein the substrate comprises a layer having front and rear surfaces,the substrate surface being identified with the front surface of thelayer, and the apparatus further comprises a backing material joined tothe rear surface, the backing material having a background color, thebackground color being any color different from the first and secondcolors and different from the colors of the third and fourth interiorsegments, and wherein:the aligning step comprises presentation of thetransparent fifth aspects of the balls of the selected array element toan observer situated favorably to observe the substrate surface in thevicinity of the selected array element, so as to permit the observer toview the backing material through the transparent fifth aspects of theballs of the selected array element.